I have used GF 8800 GS (on par with GF 9600 GT) on a Pentium D 2.8 GHz processor (915) and also on a Core 2 Duo 2.2 GHz processor (E4500). The former processor severely bottlenecked my GPU. But with Core 2 Duo, it was great experience. FPS in most games doubled, and in some games; even trippled. Now you do the math.
Any GPU above GF 8600 GT will be bottlenecked on P4 platform.