Yeah woot, I think you will be dissapointed if you upgrade now, your not going to see a big difference no matter what you get. Wait at least a year and see whats out there.
You would see a pretty big difference with a more powerful gfx card in gaming. That is definately holding you back more than your cpu at this point. Not that the 6600GT is a bad card by any means, but a 6800GT, or if you get a 7800GT, that will be huge change compared to anything you could do with your cpu.