I used the $550 build they did on tom's as a starting guide, and came up with the following:
I couldn't fit a 5770 or a 5750 into the build, not if you want the OS included in the price. 4870 should be able to play most games on high. Also replaced the cooler master psu with one of the more reliable ocz ones.
Use the coupon code EMCYWZX24 for $10 off the psu (expires 8/25/10) to make the total ~551 before rebates, $486 after.
In order to get more performance out of the rig, you can try unlocking the fourth core on the cpu with the mobo, then get a cpu cooler like the Hyper 212+ from amazon and overclock it.