#1544
I have to ask since some people here are fake flagging:
Are you in the Czech Republic? In that case your budget is probably not in $.

Absolutely sure about the RAM, but CPU depends on the game. So it would be nice which ones you'll be playing. In 99% though you'll be limited by the GPU, not the CPU. Even in TF2 which is notoriously CPU heavy going beyond an i3 won't help although that does have different reasons (spoiler alert: TF2 is a piece of shit and uses only 3 threads, so 4+ cores do nothing).

PSU depends on what's available and cheap. There's a lot of good budget PSUs but what's worth buying at 30$ probably isn't worth buying at 60$.

#1542
What are you going to use it for?
Budget?

Skylake (ix-6xxx series) is newer and faster than Haswell (ix-4xxx) and should be the same price.
16GB RAM are overkill for that build.
It would also be wise to drop down to an i3 instead of an i5 and use the money saved from that and the 8GB RAM to get a better GPU instead. That is of course if you're going to use that pc for gaming.
The PSU isn't great.

A second monitor and DVD drive shouldn't be a problem. Plug them in and done. If you want to move Windows and/or programs to an SSD later you'll have to reinstall, just copying won't work. So that'll be some work, but it is possible.

Had a talk with
#1533
on Steam, so the 4TB HDD isn't random.

Here's the partlist:
CPU: i5-6500
Mobo: ASRock B150M Pro4S
RAM: 2x 4GB 2400MHz CL15
SSD: 250GB 850 Evo
HDD: 4TB Seagate Barracuda
GPU: AMD R7 360
PSU: Corsair CX450M

total ~700€

Note about the PSU: He said he wanted a better one than the old 350W he had, otherwise I would've gone with a cheaper one. No point in getting a new one though if it's not better than the old one.

You could in theory get a cheaper mobo with only 2 RAM slots, but I think it's better to have the option to upgrade to 16GB later.
120GB SSD would only be 20€ cheaper, but you could do it to get a 5TB HDD. It's definitely noticable sound wise if you do random reads/write (otherwise it's dead silent) but it's blazingly fast because of it.

EDIT1:
#1538
Monitor:
It's not "a lightboost" it's just "lightboost". Also known as ULMB, motion blur reduction or strobing.
I'm not sure if you know what G-Sync/FreeSync do, they're quite different from Vsync.
You should probably read up on both lightboost and G-Sync/FreeSync.

IPS vs TN actually used to be the opposite of what you're thinking. TN is faster so all 120/144Hz panels were TN. You can get >120Hz IPS panels now but they are far more expensive than TN panels. The main advantage of IPS is better colour reproduction. You also get better viewing angles (picture doesn't wash out / colours get weird when you view it from above/below or the sides) but that's not really important for a monitor, on a TV it would basically be a requirement.

So yeah, read up on these things. But the XL2730Z seems like a bad choice so far, you'd be paying a lot for things you won't or can't use. 27", 1440p, Lightboost you said you don't want and even though it's got FreeSync you wouldn't be able to use it with an nVidia card.

SSD:
A faster one should be within budget but we'll see that once the time for the final partlist rolls around in a month.

WiFi:
The router got n so it makes sense but I'd still go for a slightly cheaper adapter. If you're not planning on sending files around in your network anything beyond your actual download still won't be usable anyway.

EDIT2:
#1539
After some googling I've confirmed my suspicion: The i7 will make almost no difference.
https://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/Support-for-multi-core-processors-with-AutoCAD.html
http://betterrevit.com/tips/hyper-threading-for-revit/

http://www.extremetech.com/wp-content/uploads/2012/07/3dsMax12-StarDestroyers1.png

In fact since Hyperthreading does almost nothing even a Pentium (should be almost the same speed as an i3) should be fine as long as you get the highest clockrate.

http://allcompanies.website/wp-content/uploads/2016/02/4d71cb769da8660ed87eec36c17ceca2.jpg

Judging by that ~50% slower than an i5.

I'll still have to look up how much GPU power these programs need and how much RAM would be reasonable.
Another edit will follow.

EDIT3:
#1540
Record or stream?
When are you going to build it?
How much fps would you like to get in Overwatch and Borderlands 2 and on which settings? Resolution will be 1920x1080 I presume?

Overclocking comes at a premium these days, especially Intel since they'll let you pay for an "unlocked" CPU and a chipset that allows overclocking. So the price to performance ratio will be worse. Most games are GPU limited so you'll only get better performance in TF2. And you actually have to oc. If you don't really need the performance and up just letting the CPU sit at stock speeds you'd have wasted a lot of money. Depending on your performance goals for Overwatch and Borderlands 2 there might not be enough of the budget left anyway, but if there is, would you definitely want to overclock?

Size for the SSD and HDD?

#1533
I didn't forget you.

#1535
Follow the link. You can also ask questions on /r/MechanicalKeyboards/, they'll help you out.

For the monitor:
Lightboost yes/no?
FreeSync/G-Sync yes/no?
IPS yes/no?
1080p, 1440p or 2160p (aka 4K)?

For the SSD:
Do you want/"need" a larger/faster SSD?

For the WiFi:
What speed can your router handle? a/b/g/n/ac? Just the model of the router is fine too, I can figure it out. An ethernet cable would obviously be better though.

When are you going to build?

#1536
The 6600K is still high end. Low end is <50$.
Overclocking yes/no?
Any games?
Need/want ECC?
Just making sure: Are you actually in Brazil or just fake flagging. Budget always in local currency please.

I'll have to look up the programs so I'll get to the partlist this evening at best.

#26
Not quite, you've got 3 fans. Cooler is the metal thing that goes on the CPU. There's a fan on that too. However "boxed" means that the CPU comes in box, bundled with a cooler, so you don't need to buy one.

I'd get a different PSU.
http://www.microcenter.com/product/457191/430_Watt_80_Plus_Bronze_ATX_12V_Power_Supply_Refurbished
Should be a bit better and cheaper. Or
http://www.microcenter.com/product/457217/CX500_500W_ATX12V_v23_80_Plus_Bronze_ATX_Power_Supply_Refurbished
The CX430 only got one 8 pin and that limits you a bit when upgrading/replacing the GPU.

On that note with the CX500 you could get
http://www.microcenter.com/product/451824/Radeon_R9_380_2GB_GDDR5_PCIe_Video_Card
If the rebate applies. 32$ for 30% more performance. Not necessary, it's just a nice to have.

SSD isn't important but it's nice to have, the pc will boot a lot faster, programs start faster and so on.

The motherboard is wrong though. Wrong socket. LGA1150, you need LGA1151.
Also Z chipsets are for overclocking. Since you can only overclock -K CPUs you don't need that.
http://www.microcenter.com/product/454324/GA-H170M-D3H_LGA_1151_mATX_Intel_Motherboard
I'm not sure if the combination of that mobo and the i5-6500 is eligible for the 30$ bundle rebate, but it would be nice (and then you spend those 30$ immediately on the 380, microcenter wins again).

You can order the monitor wherever you want since it's not part of the build process, right? Get the BenQ XL2411Z if you want Lightboost, AOC G2460PF if you don't.

#27
Which one?

#1532
http://www.pcper.com/reviews/Cases-and-Cooling/Be-Quiet-Silent-Base-800-Mid-Tower-Case-Review/Silent-Base-800-vs-Silent-B

#1533
I'll make a partlist tomorrow.

#20
Which one? I made a lot of builds.

There is no preference here. AMD hasn't released a new high end CPU in 4 years. And even back then the single threaded performance was terrible. TF2 only uses 3 cores (2 cores with 2 logical cores each (i3) works just as well), so you want those to be as fast as possible. So getting an AMD 8 core for the price of an Intel 4 core looks nice on paper but since 5 of those 8 cores won't be doing anything and since the cores a so slow that even if all 8 are used it can only just about trade blows with the Intel 4 core it's really not a good deal at all. Remember that was 4 years ago.
Sure, Intel hasn't gotten a whole lot faster but neither has AMD. So while AMD does sell some nice cheap overclockable Quad Cores (Athlon X4 7x0K and 8x0K) even overclocked they're still slower.

#21
You've been doing something wrong then. Intel doesn't do price drops and they always release new CPUs that are a few percent faster at almost the same price. No reasons to buy it if it were 50% more expensive.

You'd have to triple the 380's power draw to max out that PSU. I'm pretty it would catch fire way before that.

You wouldn't buy the XL2411T either. Same panel, but the Z offers more/better features. Also the colours on the Asus are worse. I lightboost they are just terrible. I don't mean "you can see it next to an IPS panel" terrible. I mean "why is this blank white page red" terrible.

#22
What if I told you neither is worth buying?

#1530
https://tweakers.net/pricewatch/353438/corsair-graphite-230t-windowed-orange.html
No sound dampening though.

https://tweakers.net/pricewatch/464664/be-quiet!-silent-base-600-zwart-oranje.html
Less orange, but with sound dampening.

https://tweakers.net/pricewatch/448640/be-quiet!-silent-base-600-zwart-oranje-window.html
Same but with window.

#11
For TF2 you can get away with an i3, an i5 makes almost no difference (5%). I think CS:GO needs a bit more power so to be on the safe side you want an i5-6500 or i5-6600.
I'm guessing you need a monitor as well? Is that included in the 100$ budget?

Would be good if you could post whatever partlist the shop suggest so we can check it.

#17
Wouldn't recommend Haswell anymore, Skylake is faster and the same price. Same for DDR3 vs DDR4.

Also mediocre overpriced 750W PSU for a <400W, smh.

I get tired of saying it but the XL2411Z is the same price as the VG248QE and far better.

#1527
What case? Need to know for size?
8 or 16GB RAM?
Rough idea how much GPU power he needs for those games?

#1528
Mobo size (ATX or µATX) and cooler height (or model).

Some general idea what you're looking for in terms of design.

Sound dampening yes/no?
Budget?

#1525
I think the obvious thing to improve would be the RAM. It's the reason why pcpartpicker isn't showing the mobo. It's DDR3 but the mobo (and the CPU, at least officially) only support DDR4. The CPU not supporting it is more of a legal thing, Intel also doesn't support overclocking yet they happily sell overclockable CPUs for a premium. The mobo however is physically incompatible. DDR3 won't fit.

Next up: The 212 Evo.
WHYYYYYYYY?!
http://www.teamfortress.tv/post/626834/computer-has-lost-its-processing-power
Am I stuck in the fourth circle of hell?

But I'll repeat it once more:
The 212 Evo is a budget cooler. When it's not on sale there's cheaper coolers that perform the same or better.
Outside of the US it is never on sale that cheap.
When you're not looking for a budget cooler there far better coolers.

So lets see: You're outside of the US, have virtually no budget constraint on the cooler and want to overclock properly (I guess?). Yeah, the 212 Evo makes absolutely no sense here.

You could get something faster and/or larger than the 850 Evo 250GB. Sandisk Extreme Pro is faster and the 850 Pro is even faster than that.

Why two 1 TB HDDs instead of one 2 TB HDD? Why the WD Blue? More expensive, older and slower than Seagate's or Toshiba's alternatives in that price range.

1070 is newer and faster than the 980 Ti unless I'm terribly mistaken. Same price so no reason for the 980 Ti.

A mediocre 750W 80 Plus Gold PSU isn't what you want for a 450W build. Especially when better ones are cheaper or at least the same price.

So here's some improvements:
PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i5-6600K 3.5GHz Quad-Core Processor (£210.00 @ Amazon UK)
CPU Cooler: Noctua NH-D14 65.0 CFM CPU Cooler (£56.90 @ Amazon UK)
Motherboard: ASRock Z170 Pro4 ATX LGA1151 Motherboard (£90.99 @ Amazon UK)
Memory: Team Vulcan 16GB (2 x 8GB) DDR4-3000 Memory (£62.60 @ Amazon UK)
Storage: Samsung 850 EVO-Series 250GB 2.5" Solid State Drive (£72.99 @ Amazon UK)
Storage: Toshiba 2TB 3.5" 7200RPM Internal Hard Drive (£56.97 @ CCL Computers)
Video Card: Palit GeForce GTX 1070 8GB GameRock Premium Video Card (£406.98 @ Ebuyer)
Case: NZXT S340 (Black/Red) ATX Mid Tower Case (£64.99 @ Overclockers.co.uk)
Power Supply: EVGA SuperNOVA G2 550W 80+ Gold Certified Fully-Modular ATX Power Supply (£70.97 @ Amazon UK)
Wireless Network Adapter: TP-Link TL-WDN4800 PCI-Express x1 802.11a/b/g/n Wi-Fi Adapter (£24.93 @ Amazon UK)
Monitor: BenQ XL2730Z 27.0" 144Hz Monitor (£435.50 @ Kustom PCs)
Keyboard: Corsair K70 Wired Gaming Keyboard (£89.99 @ Amazon UK)
Total: £1643.81
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-08-13 16:46 BST+0100
Didn't change the SSD yet.

WiFi and monitor depend on what he needs/wants. Can figure that out later for him but I'll have to ask him some questions (same for the SSD).

There' usually better/cheaper options than Corsair for mech keyboards.
Again, depends on what he wants.
https://www.reddit.com/r/MechanicalKeyboards/wiki/buying_guide

Also with some minor changes you could add SLI capabilites to the build above
http://uk.pcpartpicker.com/list/4wVfPs
RMx 650 because it's slightly better than the G2 650 and only 2£ more.

Different set of the same brand or different set of a different brand makes no difference.
Either way memtest86 would still help.
It would rule out windows if there's still errors.
If memtest also produces errors with a different mobo then apart from some really weird PSU shenanigans it could only be the CPU.

I thought I'd be able to post something here "today" but since it's technically already the next day and I'm tired I'll just call it a night.

Don't worry, I'll post something tomorrow.
#8 is solid advice though.

But which one? Tell me your secrets!

Already tried running memtest86 for a gazillion years with both sticks? If either one is actually bad there should be at least some errors.