Well, my old AM2 socket is pretty much maxed out.
All RAM slots filled.
GPU is really only thing I would want to upgrade :-/
Ok, well, you are using DDR2 ram, right?
You could go for an AM2+ MB, reuse your old ram, get a phenom II 955 and an HD5850/5870....
That would do more for your system than only getting a 5970 and it would be cheaper as well..