Why do you want to upgrade? What games are you playing that you do not get the desired performance?
If anything, Xfire 5770 = 5870, so yes a 6950 or 6970 would be an upgrade.
There is nothing wrong with your CPU at all, you should do a little reading up on Overclocking so you can turn that CPU up a little, What cooler are you running, if its stock, upgrade that ASAP and Overclock that CPU to at least 3.4GHZ
If you don't have a SSD, that would probably the most noticeable upgrade to the system as a whole, it wont improve your Game FPS, but it will improve the responsiveness of the PC and Load times of games!