In case anyone is interested, I decided to turn this into a gaming rig. I followed a lot of the information in this thread, and also researched all the recent GPUs. I've never had a gaming computer before so I knew I'd have to take some time to figure out the best bang for my buck. After all, this rig started as an affordable commodity.
Like what was stated in the thread earlier, all that is necessary is a PSU and a GPU and you got yourself a decent gaming rig. I didn't really know what would fit but found that the case is just a mid-tower and any standard PSU will fit.
For the PSU I went with the Rosewill Green Series RG630-S12 630W:
I'm not sure what makes a PSU good, but it got good reviews on NewEgg (4 eggs with 200 reviews) and it was cheap @ $60. Plugged it in, worked well, had plenty of power connectors and the fan is very quiet. Very happy with this purchase.
For the GPU, I went with the SAPPHIRE Radeon HD 6850 1GB 256-bit GDDR5:
It was only $150 ($160 after shipping + tax) and was ranked really high on videocardbenchmark.net:
It is ranked 13 out of around 200 of the best high end graphics cards there. It also got best bang for your buck on tomshardware for the $160 mark for September 2011.
I installed the PSU and GPU on Saturday and everything worked the first time around. Super simple. I was slightly worried that the GPU (which takes up 2 slots) might be too big for the mid-tower. Or at the very least I'd have to rewire some of the internals to accommadate it. But needless to say, the card popped in perfect.
So after both were installed I wanted to try it out.
I downloaded the Battlefield 3 beta to not only play an awesome game but to test. The game automatically assigns everything to "high" for the card. Although, there is an "ultra", i put it on it and it ran fine.. i seriously could not tell a difference between high and ultra. Fraps tells me I get about an average of 42 FPS on "high" setting. The game runs flawless.
I mainly wanted to get the PSU and GPU upgrade for Diablo 3, but now that this thing runs great I think I will be buying Battlefield 3 because it is one of the best war games I've played (even though it's still in the beta!)
So after all is said and done the money I spent was around $500 for a very good gaming rig. It plays the latest high graphics intensive game (Battlefield 3) on high at 40FPS. Spending any more money on a graphics card would just be future proofing it for other very high graphics intensive games. Diablo 3 will have lower requirements than Battlefield 3. The 4gb of memory is more than enough.. the quad core is more than enough.. and the graphics card is more than enough.
Computer - $305
GPU - $160
PSU - $60
525 for a great gaming rig. Definitely worth the buy.