you're not gonna get much bang for your buck with a prebuilt, the Dell XPS gaming systems start at $1100, and that doesn't include a monitor, also that's with just an HD3870, with a decent gaming video card (8800GT) the price is $1200, and that's with a 320GB hdd, and 2GBs of PC2 5300 (only 667mhz, PC2 6400 is dirt cheap right now and is 800mhz)...
if you built a similar system yourself, it would probably be no more than $700-$800 tops...