The 7600GS is noticably slower and less capable than the 7800GS, let alone the 7900GS. Definitely go 7900GS if you're deciding between the two.
The memory doesn't really matter. If you have 256 and a fast chip, that's all you need. The common trend is to throw cheap memory onto an older card to make folks think it's better (because it has more RAM), but that doesn't make it crunch frames any better.