Gaming computer for cheapness

Solution
I've been in and out of the other two sites a few times and haven't been able to keep your budget. Looks to me like the Killer Gaming V is your best option
Starting with your prebuilts...
1) Dell OptiPlex - Best part, the price. Comes with Win 7 32bit which limits the machine to 3.5GB of ram usable (not good), an 80GB HDD which windows 7 takes 1/4th of (again not good) and a Radeon HD5450 which is considered anemic by current standards. One of the biggest drawbacks though (for gaming) is the dual processor setup, many games are not set up to take advantage of multiple cores, let alone multiple processors. As good as the price is, I'd pass on that one.
2) KILLER GAMING PC! V - looks to be a major step up from the OptiPlex option and about my only concern with it is the generic PSU that comes with it. I would've liked to have seen a 7770 rather than the 7750 though but, not too bad for the price
I've run out of time right now but I'll see if I can't throw together a decent configuration from the other links later