So, I'm thinking to myself, I want a great gaming rig at a low price, seems I found it when toms crew came up with their $650 gaming computer, but they seemed unhappy with some items and mentioned they would like to upgrade if they had the added cash for the build.
Phenom 720 X3 black edition, which is only $10 more now than the dual core they used.
And it is a bit of a stretch over your $50 limit, but I would get a single 5850 for around $259.
I am not sure how much "better" that would really be, but I would at least go for the triple core CPU, and if you can swing it, you have DX11, and a new generation card that you can add another to later down the road for crossfire. Thats my 2 cents anyway.
I agree changing to the Phenom II x3 720 BE would be an improvement. Also, I would switch out your hard drive to either the Western Digital 640GB Black or Samsung Spinpont F3 hard drives. Both are a lot faster than the one in the build. Switch the Case to the Cooler Master RC-690.
Here is the list... Plus $49 dollars over the Tom's $650 build...