All I can offer is an educated guess, if you want precise numbers you should ask someone with a similar setup to test those settings. I've recommended a lot of similar builds so you shouldn't have any trouble finding someone who could help you with this.

I was assuming you were going for the usual fairly low settings in CS:GO since you were going for a low midrange GPU. CPU wise 400-500fps wouldn't be a problem. But for something like 1920*1080, 16x AF, 8xEQ AA/16xQ CSAA a 270X will limit you to 150-200fps. Not really a problem on 60Hz, but it could be less than optimal if you want to get a 120/144Hz monitor down the road. It's obviously not a problem if we're talking about more reasonable settings like 8x AF, 8x MSAA, that should put it at 200+ fps.

If you're sure about max settings I'd either take the CPU down a notch (back to an i5 again) and get a better GPU instead or I could squeeze in a 290 or even a 970 regardless. That should put you in the 300-400fps range.

It's just about the driver. AMD Gaming Evolved is like Geforce Experience, you don't need it, unless you want to use some of its features.
If the driver version has changed to whatever you tried to update it to, then it worked.
Here's the link to the driver download page, just in case.
http://support.amd.com/en-us/download

It's going in the right direction but there's so many textbook mistakes, I just can't resist tearing it apart.
It might help whoever reads this to avoid them.

CPU: Outdated, the i5-4590 is faster and only 2$ more.
Cooler: Why? 1. There's no point in buying the 212 Evo for 30$. It's a bargain at 20$ and ok at 25$, but at 30$ there's better coolers. 2. The i5-4570 can't be overclocked so you don't need an aftermarket cooler.
Mobo: Same thing, can't overclock anyway, why get a Z87 motherboard?
RAM: Specs are fine, but you can get the exact same specs for 10$ less.
HDD: Seagate Barracuda > Western Digital Caviar Blue. Identical Specs, slightly faster, 1$ cheaper.
no SSD, your choice but it would've fit the budget and imho an SSD is worth it.
GPU: Unless you absolutely need nVidia, the low power consumption (not on a desktop except for compute) and that exact performance the 750 Ti isn't worth it. There's better or cheaper nVidia cards and better and cheaper AMD cards, only the power consumption is hard to beat in that performance class.
Case: It doesn't even have USB 3.0. Outdated and lacks features (e.g. dust filters).
PSU: The CX430M is a budget PSU. See Cooler. It's a bargain at 20$, ok at 30$, but at full price (60$) or close to it (50$) it doesn't stand a chance. There's so many better PSUs at that price. You might consider it at 40$ if you want a semi-modular PSU and there's no better options, but at 50$ when the non-modular CX430 is at 20$ it just doesn't make sense. Even the CX500M is 6$ cheaper. The CSM450 is 6$ more for that matter. So there's an identical non-modular PSU for less than half the price, a higher wattage and otherwise identical PSU for less and a way better PSU for 6$ more. Yeah, no point in getting the CX430M.
ODD: There's cheaper with the same specs. It's not a whole lot of difference, but 5$ is 5$.
OS: You are aware that Windows 10 will be a free upgrade even for the ... uhm less legitimate copies of Win7/8. Of course I do not officially condone this, just like Microsoft.
Monitor: Anything specific you're looking for?

That's it for the destructive criticism, here comes the constructive criticism:
Fixed everything I complained about, upgrades to CPU and GPU, added SSD.
PCPartPicker part list / Price breakdown by merchant

CPU: Intel Xeon E3-1231 V3 3.4GHz Quad-Core Processor ($241.99 @ SuperBiiz)
Motherboard: ASRock H97M Anniversary Micro ATX LGA1150 Motherboard ($68.99 @ SuperBiiz)
Memory: Pareema 8GB (2 x 4GB) DDR3-1600 Memory ($54.98 @ Newegg)
Storage: OCZ ARC 100 240GB 2.5" Solid State Drive ($80.98 @ Newegg)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($49.99 @ NCIX US)
Video Card: PowerColor Radeon R9 270X 2GB PCS+ Video Card ($143.98 @ Newegg)
Case: Cooler Master N200 MicroATX Mid Tower Case ($43.98 @ NCIX US)
Power Supply: Corsair Builder 430W 80+ Bronze Certified ATX Power Supply ($19.99 @ Newegg)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.99 @ Micro Center)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($89.99 @ NCIX US)
Monitor: BenQ GL2460HM 60Hz 24.0" Monitor ($138.99 @ NCIX US)
Total: $948.85
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-03-23 06:24 EDT-0400

Well of course they recommend a new PSU. Troubleshooting software is more work, they might have to face angry customers because they changed some ultra important settings (which might have been causing the problem). If they just sell you a new PSU it's almost none or no work at all, they make easy money and if it still doesn't work they can quote the official requirements and tell you, that you needed that PSU anyway.

If you can return the CX430, you could get an EVGA 500B from CPL for 78$. Let's see how fast they backpedal once you tell them you won't be buying the new PSU from them.

All profiles are shit, there's usually only one decent profile and that's either standard or sRGB. For the Benq it's standard.

You know that the ICC profile might be responsible for the too bright low blacks. They might be ok with lower brigthness, but you turned the brightness up.

You can't turn Black Equalizer off?

Using an ICC profile with completely different settings than what were used to create it actually makes things worse.
Disable it and take your time and configure the colours manually. I'd lend you a colorimeter but you don't exactly live next door.

#6
ICC = colour profile.

It would've been a lot easier if you had bothered to provide a link.

Those profiles will also completely fuck up the colours. They're only useful for two things: Setting the motion blur reduction utility settings and black equalizer for different games. The XL2420T isn't compatible with the mbr utility so you'd go through all this trouble for a single setting (black equalizer) for which you could find your preferred setting in five minutes.

It's easier to creat your own profile.

#8
You don't like brightness, you don't like the contrast and you're probably just telling yourself the colours are better. That ICC profile is useless for you. Borrow a colorimeter and make a proper ICC profile that actually improves things. Or tweak the OSD settings manually, you can actually get acceptable results with this monitor.

So we're not talking about ICC profiles since those aren't personal preference.
What kind of profile are you looking for?

That driver is fairly old. Try 14.12 (Omega), maybe the 15.3 beta after that.

What made you choose the XL2420Z over the XL2411Z?
What kind of profile do you need? Downloading an ICC profile is pretty much pointless since it won't fit your specific panel. If all panels were created equally we wouldn't need ICC profiles.

What's your budget?
And yes, it should be fine.

Just some suggestions:
Imho overclocking isn't really worth it at the moment. I mean you're looking at +10-18% single threaded vs a Xeon E3-1231 v3 depending on the silicon lottery. There's not many situations where that would matter. If a game is single-thread bottlenecked by the CPU 100% of the time it has no multithreading or very poor optimization. If it's an old game you'll get a gazillion frames anyway, if it's a new game with ubisoft optimization going from 30 to 35 fps isn't going to help much.
Sure, fps drops cause by a single threaded bottleneck would be worse (e.g. 50 instead of 60fps) but let's be honest: You'll need a lot more GPU power than a single 970 for that to happen. I mean you could force it with 1280*720 on low settings, but on 1440p? Not happening.
And once we get into multithreaded, streaming is the best example, the i5 will lose every single time. Overclocked. On stock it'll just get wrecked.
Not even counting the mobo i5 + cooler is more expensive aswell.

RAM: Although the difference should be negligible dual channel is definitely faster. And it's a whopping 0.01$ cheaper.
HDD: Seagate Barracuda. Same specs. Slightly faster. Slightly cheaper.
PSU: No good deals right now. CSM450 is the best I can do. I'm not going to recommend the Rosewill ARC series until I've seen a review for it. The platform is ATNG ATM-B, the same as in the Coolmax ZX-500. I mean it's better than the usual stuff from Coolmax. It doesn't chatch fire and if you pull the rated wattage only a resistor and the PWM controller explode instead of basically the whole unit. Performance was only below average and not horrible so if they used proper components it could potentially be decent, but even then it would still be a group-regulated PSU without all those pesky features like OCP that prevent things from catching fire and/or blowing up which places it firmly into the shit tier of PSUs until a review proves otherwise.
ODD: Double the buffer and can write blu-rays. I agree with your choice if you won't ever write blu-rays.

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Xeon E3-1231 V3 3.4GHz Quad-Core Processor ($241.99 @ SuperBiiz)
Motherboard: ASRock H97M Anniversary Micro ATX LGA1150 Motherboard ($68.99 @ SuperBiiz)
Memory: Pareema 8GB (2 x 4GB) DDR3-1600 Memory ($54.98 @ Newegg)
Storage: Crucial MX100 256GB 2.5" Solid State Drive ($104.98 @ NCIX US)
Storage: Western Digital Caviar Black 1TB 3.5" 7200RPM Internal Hard Drive (Purchased For $0.00)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($49.99 @ NCIX US)
Video Card: MSI GeForce GTX 970 4GB Twin Frozr V Video Card ($326.98 @ Newegg)
Case: Cooler Master N200 MicroATX Mid Tower Case ($43.98 @ NCIX US)
Power Supply: Corsair CSM 450W 80+ Gold Certified Semi-Modular ATX Power Supply ($55.98 @ Newegg)
Optical Drive: LG WH16NS40 Blu-Ray/DVD/CD Writer ($49.99 @ Newegg)
Total: $997.86
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-03-20 13:27 EDT-0400

GPU confirmed. My bet is still on software for now.

Up-to-date BIOS != working BIOS. No correlation there, old BIOS can be good and new ones can be faulty.

What about the drivers?

If it still doesn't work with the newest drivers then the last thing you can try is flashing the BIOS or if you're not comfortable with that or if it doesn't work either, RMA it (after flashing back to the original BIOS).

From now on anyone who says "TF2 is dead/dying/ded" has to pay me 1$/€/£. I'll get filthy rich in no time!

There have been far worse things than Froyotech and either they didn't kill TF2 so Froyo won't either. Or TF2 got killed long ago and we just didn't notice. In that case Froyo isn't a thread anymore.

Dignitas didn't kill TF2.
Old Epsi didn't kill TF2.
New Epsi didn't kill TF2.
i46 mix^ didn't kill TF2.
i49 Epsi didn't kill TF2.
i52 Froyo won't kill TF2.

Hell, if Defcon 0, fakkelbrigade winning prem, couldn't kill TF2, nothing will!

Worked out the details via steam.

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Xeon E3-1231 V3 3.4GHz Quad-Core Processor (€248.20)
Motherboard: ASRock B85M Pro4 Micro ATX LGA1150 Motherboard (€58.99)
Memory: Crucial Ballistix Sport 8GB (2 x 4GB) DDR3-1600 Memory (€59.05)
Storage: Crucial MX100 256GB 2.5" Solid State Drive (€96.99)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (€49.80)
Video Card: XFX Radeon R9 280X 3GB Double Dissipation Video Card (€225.99)
Case: Cooler Master N200 MicroATX Mid Tower Case (€33.09)
Power Supply: EVGA 500W 80+ Bronze Certified ATX Power Supply (€43.99)
Optical Drive: Lite-On iHAS124-04 DVD/CD Writer (€11.17)
Total: €827.27
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-03-19 18:39 CET+0100
Current prices from mindfactory and kepcom. Best case for shipping (midnight shopping at mindfactory -> free, 4.99€ for kepcom) would put it at 834.26€ total.

Links:
Xeon E3-1231 v3
ASRock B85M Pro4
Crucial Ballistix Sport 8GB
Crucial MX100 256GB
Seagate Barracuda 7200.14 1TB
XFX R9 280X Double Dissipation
Cooler Master N200
EVGA 500B
Lite-On iHas124-04

Don't worry there's throttling and thermal shutdown to prevent that.

Glad that fixed it.