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$$.
Either way, they are both decent cards.