I included a GPU in there, too, a good one, and it's still under $600 total (well including mail-in rebate, $624 without mail-in rebate).
If you want one that's a little better...
$265 before mail-in rebate[/url] With that instead of the 5770, the pre-mail-in rebate total would be $749.
$213, a little less powerful than the 6870, but still more powerful than the 5770.
If that's still a bit much, you could use the GTX 460 768MB
which is $167 before mail-in rebate, but $137 after mail-in rebate, and it's a bit better than that 5770.
(The * sign in the prices on that site mean after mail-in rebate)