To get any significant upgrade right now, you would have to start with a 5870, or CF 5850s. Those cards can be tough to find right now, and are carrying inflated prices. If you are already getting enjoyable performance from your set up, you may not experience any real gain from the investment. If you go with Nvidia right now, you will not have DX11, and will pay even more for the same graphics power.
I say wait until you start feeling your cards choke on the games you play, or you get desperate for DX11 goodness. By that time, there should be better options available.