The 8800GTS 640MB will give you better performance especially @ 1920 x 1200 resolution. Definitely not worth upgrading to a 9600GT.
The 9600GT was and still is a good mainstream card. It's main competitor is the Radeon 4670; which basically trades blow for blow in games making them equal overall. The 4670's edge is that it only uses about 47w of power, roughly 13w less than the 9600GT.