Nice try, guys, but the 8600 and 9600 are different, it is not quite like the 8800/9800/GTX250 mess.
OP; No it will make no difference, only a few of the high end X2 cards can get close to using all the speed of PCI-E 1.0, two 9600 will hardly dent it.
Here is a review of the 9600GT which benches it alongside the 8600GT:
http://www.trustedreviews.com/graphics/review/2008/02/22/nVidia-GeForce-9600-GT/p1