First of all, the 580 has been discontinued by Nvidia and it's pricing are outdated and really irrelavent.
the 660 is rumored to be priced around $300, just over your $275, and should perform above a 7850. how much above, no one really knows. given how the 670 is encroaching on Nvidia's 680 sales, I doubt the 660 will be an amazing performer like the 670. but it should still be faster than the 7850
EDIT: sorry, forgot to mention, as far as performance goes, I think Nvidia will try to shoot for 580 performance. but it could end up slightly slower, we don't know for sure unless there's more details