Looking at tom's charts, with overall fps, it does not have the 9600gt. So, what I did was I looked at the 3850, and the 8800gt.
The 3850 is about the same as an 8800gs, which is a little less than the 9600gt. The 8800gt is a little more than the 9600gt. So the 9600gt would be about right in between the 3850 and the 8800gt. And that's where the 8800gts 640 is.
So really, they're about the same. Maybe keep the gts, because it has more memory which might help later. Or maybe pick the one that is the quietest/coolest.
I don't know why, but I think I'd like the 9600gt better.
Sell both, and upgrade to a much better card for your new rig.
Actually, that might not be a bad idea. You might be able to then afford an ati card or another nvidia card.
IMO, I would stick with the 9600gt since it is a newer tech and it works perfectly fine for me.
I don't know the tech specs for comparison, but i'm sure both are quite close to one another. however, i do know that two 9600gt's in sli will kick a 9800 series in the a$$.
if you sell em both you may be able to afford a 4850/4870. I have no idea how much they sell for so maybe not. but i used to have a 8800 and my friend CURRENTLY has a 9600 and the 9600 is alot smaller, cooler, and is just a better overall card in my opinion. they trade blows pretty evenly performance wise, except his 9600 runs crysis better