Re: Upgrade from 5850: which one?
570 and 6970 are about even. There may be a few titles that favor one over the other. Considering the 570s are a little cheaper right now I'd recommend that. 580s are just too expensive to be a good value, but if you want the best possible single gpu performance that's your only solution.
If you can find a very cheap 5850 and compatible PSU, crossfire actually might be a better solution. In applications that scale well with crossfire, your framerate will likely be higher than a single 580.