As AMD Man says, why you need to upgrade? The only reason that make sense for me is that you are playing games at high resolution with all enabled. If this is the case, its clear that the most benefit will come from a new video card.
But for me, still is not worth the expense ...
Note: you can check THG graphs to see the improvement from GF3Ti500 to GF4.
DIY: read, buy, test, learn, reward yourself!