Hmm, I honestly have no idea. I think they would be on par with each other. If it was the 768MB version, it would outperform. I'd just go for the one you're comfortable with.
At lower resolutions it probably would perform about the same maybe a little faster but at the high ones the 9600s extra memory and stream processors give it an edge
with some aftermarket cooling and a little overclocking you could easily outperform a 9600gt stock though.
they both are about the same but the 9600 gt will be better because it runs cooler newer processor so that means that beter drivers are coming out and also it has agivea physics technology and pci 2.0(when drivers mature for pci 2.0 it will be fast has ass)
9600 GT also consider the 9800 GT its only 109 dollars + call of duty for free