I've had a 4800+ X2 s939 for ages, and it worked rather well. building a computer when you wanted to take a small step, but might end up buying a whole new pc always is kind of a problem.
when you want a faster cpu than a 4800+, you would need a new motherboard, and new RAM (and the processor ofcourse). so my suggestion would be to wait a bit longer, and get something like a 7900gtx or 7950gt or something liek that 2nd hand.
A lot of people are ditching those cards to get a 8800GT, so i think now is a good time to get a high end dx9 card cheap

(at least over here it is)
by the time that card becomes really obselete, your processor and RAM will be too. and that would be a perfect time to replace the whole lot at once then
its just an idea, and its most likely what I would do
