Like most posts. 256 megs should be enough for up to 1280x1024. 512megs for up to 1920x1200, 1024+ megs for 2560x1600+. Anti Aliasing also uses a lot of video RAM, so you may want 512m for 1280x1024 if you do a lot of AAs.
Don't get below 256, even if you're just using Aero. I personally use Aero with 64megs on my notebook which usually runs fine. - except for running Aero with lots of windows open or running video in a window.
I still suggest the GT 512 or GTS 512 - Even though the slower/cheaper 9600GT will blow the doors off your old 7600. Newer games will need the added speed and memory.