What are you expecting this PC to be able to do after the upgrade? (like say, "run modern games on med/high settings at 1080p resolution")
Although powerwise you might be able to squeeze in a GTX 670 ($400)into that PC, by the DDR 2, I am assuming you have a Core 2 Duo. That processor will not allow that card to reach its max potential in some games. But will still take you as high as your PC will allow.
Stepping down to a say AMD 7850 or a 560 ti card (both $250) should about match your PC to the graphics... all for a similar power draw will sacrifice a little bit of performance, but the price is much more reasonable.