Yeah, that's worth it and hell of a good deal.

If you don't have a name brand 500W power supply, you should get a new one.

i7 920, when overclocked, will use quite a bit of power so you should have a good one if you take my recommendation to push your overclock further.

I recommend you get a 600W (or greater) from a reputable brand. Corsair, Antec, Seasonic, XFX, PC Power and Cooling, OCZ, Silverstone, are a few. Power supplies from other manufacturers are hit or miss depending on the model, and even Corsair has a range of quality units. Their 600W Builder series aren't as good as their Enthusiast series.

So upgrade the power supply, push your CPU overclock further, aim for 3.4 GHz, get a new heatsink if you must, and upgrade the video card would be the best upgrades you can do. If you feel like reformatting, pick up an SSD as it'll decrease load time tremendously.

What tooth said.

G400 is great for the price. The software is also good - you can customize every button per game.

But my first G400 only lasted 2-3 months. It would intermittently disconnect/reconnect a few times an hour when gaming. Logitech sent me a new one, for free, and it's lasted a good 10 months so far. The old one still works, but it's not viable for gaming.

Performance is good, features are good, but the build quality is right in line with the price you pay. Just comparing the cord of the G400 to my old MX510 shows a noticeable difference. The G400's is thinner.

FX-6300 should only be like $10 more. Well worth the upgrade if you can still change it.

You do a CPU, PSU, and GPU upgrade and you'll have something as good as buying a whole new machine. Just need to make sure the case is big enough/isn't specialized to accomodate a new PSU and GPU.

You don't need to overclock to get 144 fps with an i5. The DX9 config will give you that in competitive play. On large pub servers it may drop, but it will still be above 100 most of the time.

You might be able to get by upgrading your current processor. What do you have? I'm assuming you probably have a Core i3. Intel has stopped making 1156 processors, but you can readily buy a used quad core i5 or i7 either from ebay or various tech forums.

The power supply will definitely need to be swapped. You didn't list your processor. And if your case isn't low profile or too small you should be able to fit current mid range video cards (like the 7850).

I would have to look at some pictures of the outside and inside of your case, and possiby have you take measurements, to get a complete picture of what's possible to upgrade your current PC.

If that doesn't pan out, I'd recommend you bump the new system processor to an FX-8320 or Core i5. Or just bump down to an FX-6300 processor. Also to maximize your budget, since it is pretty tight, keep an eagle eye out for Newegg's combo deals and Shellshockers. For instance that 600W Corsair PSU is a great deal, but looks like it's out of stock at this moment in time.

Go with the MSI Z77a-G45 board. It should be fine. Better than the first board, but of course not as good as the Gigabyte board I recommended.

The HAF 912 is a great case, especially for that price.

It looks like the monitor comes packaged with a dual link DVI cable (you need dual link DVI for 144 Hz, single link won't work). But a DisplayPort cable will also work, so if you don't want to take the chance a DVI cables doesn't come with the monitor then buy one. You can get cheap cables at monoprice.com:
http://www.monoprice.com/products/subdepartment.asp?c_id=102&cp_id=10246&cs_id=1024601

With power supplies you basically get what you pay for - ignoring flash deals/promotions.

Modular power supplies will carry a bit of a price premium (good sales negate this premium, though). So if you're strictly looking for performance then non-modular power supplies will typically offer the best value.

So out of those listed avoid the Raidmax. It won't be very good.

The best value out is the Corsair TX 650W with the $20 off promotion. I recommend you grab that power supply before the promotion ends. You won't find a better power supply for less money.

The alternative theoretical build will perform better.

However, as mentioned, the power supply is clearly inferior. But there's an easy fix to fit a good $50 power supply into the budget by downgrading the heatsink, getting a good deal on a 120GB SSD, and making other tweaks to the choice of RAM and case.

Overclocking, fps config (lower sound quality) will definitely improve performance. The DX9 config helped the framerates on my AMD rig and the game doesn't look much different.

Also if you decide to overclock don't overlook Northbridge overclocking. Phenom processors respond well to an increase of Northbridge speed for gaming. Stock should be running at 2.0 GHz. Try to get the NB at least to 2.6 GHz, topping out at 2.8-3.0 GHz.

I would recommend you downgrade the PSU and upgrade the motherboard. 500W is enough to cover your proposed system with overclocking. 650W is enough to cover overclocking and SLI, although I don't particularly recommend SLI. Now if you weren't on a specific budget sticking with the 750W unit is fine.

Rosewill CAPSTONE Series CAPSTONE-650-M 650W ATX12V v2.31 & EPS12V v2.92 SLI Ready CrossFire Ready 80 PLUS GOLD Certified Modular Active PFC Power Supply

The MSI motherboard is compatible, but if you're serious about overclocking you should step up. More expensive boards also usually come with more features, such as more USB 3.0 ports. So it's up to you if you really want to pay for more overclocking options and more features, and here's my recommendation for a better board:

GIGABYTE GA-Z77X-UD3H LGA 1155 Intel Z77 HDMI SATA 6Gb/s USB 3.0 ATX Intel Motherboard

I don't think you should downgrade the hard drive.

FX-8350 is a fine processor, but it is overall inferior to Intel processors. It does shine in highly threaded workloads. The results of those tests are very questionable, and they also use a very questionable motherboard for the Intel tests.

If possible get a quad core i5 processor. They are worth the extra money over an i3.

I suggested you return the processor and upgrade to the FX-6300. It's a little bit more, but well worth the extra cost. It has more cores plus higher performance per core with comparable power consumption.

Actually you should read this.

http://www.overclock.net/t/946407/amd-motherboards-vrm-info-database

Your motherboard is only rated for 95W processors. If you overclock your current processor you will exceed the 95W and increase your likelihood of physically damaging the motherboard by a significant amount.

I recommend you do not overclock. But if you want to, I highly recommend you put heatsinks on the VRMs or simply get a better motherboard. But swapping out motherboards isn't as good of a value as getting an FX-6300 95W processor, which will give you better performance than overclocking your 4170. In fact, if you still have the option of returning the 4170 I would recommend you do that and get the FX-6300. Not only does it give you two extra cores, but it is an updated architecture over the 4170.

That can run TF2 above 30 fps, and well above if you use the DX9 config.

But it looks incredibly overpriced for the hardware you're getting. I wouldn't recommend buying it; you can get something better for the same price I'm sure.