Here is a great article I wanted to bring attention to titled How to Build a Kick-ass $800 Gaming PC which I found on MaximumPC. Generally if you are going to be buying a PC of any sort, you are going to want to order all the parts and build it yourself to get the most bang for your buck. Buying pre-built and configured PC's may be easier often times you will find that the computer doesn't do things quite how you would like. Building your own rig ensures that your computer is built just how you need it.
In any case, here's how Maximum PC's $800 rig stacks up in terms of gaming performance.
We went into our gaming benchmark with low expectations from our budget card, the Radeon HD 4850. Obviously, it is no match against the dual GeForce 8800 GTX setup in the Zero Point system. With settings cranked up to the max, our card was barely able to spit out 16 FPS in Crysis. While playing Crysis at the highest settings possible and a resolution of 1920x1200 simply isn’t an option, turning down the graphic settings to medium resulted in 43 FPS made the game much more playable. Unreal Tournament 3 managed to give us a stellar 78 FPS. If you’re running at typical 22-inch LCD resolutions, this machine should kick ass.