My reading of the HardOCP's CPU scaling article from a few months ago (go look I'm too lazy to find it for you

) is that the 3000+ is
not going to bottleneck much if at all with a 7900GT in most games.
Yes an entirely new core2 / 8800gts system would be nice, but it doesn't sound like that's in your budget.
Also, what specific games do you want to play? 2GB helps some, not others.