how much better is a 7900gt than a 7600gt

how much better is a 7900gt than a 7600gt is it about 30 fps better or so?

A lot better.

Depending on what you're playing, you're $100 might be better spent on an extra 1gb RAM.

Exactly my point.

Let's say you have X2 3800+, with 2gb of RAM, and a 6600GT. The 7900GT is a very HUGE, and worthy upgrade. But if you have that system with only 1gb of RAM, it can bottleneck the 7900GT in some games. Making it a waste.
i guess the difference would be the hiccups when the game is swapping memory with the disk, but i dont think 1GB would bottleneck a 7900GT. The in-game smoothness would be a lot bigger with let's say a 7600GT and 2GB then with 1GB and a 7900GT. All depending on the game, of course.
In my opinion, if i had 1GB i would go for the 7900

edit: http://www.tomshardware.com/2005/1 [...] age12.html

u couldnt put a fps number on the difference coz it is always different
if u want to compare by numbers u should gav asked "aprox wat % of fps more would it get"

there are plenty of benchmarks including both cards here at TH or if not try anandtech. The 7900gt is definetly worth the extra $$$ if u can afford it.