the 660 is rumored to cost $300. as for it's performance, my money's on it being on par as the 580, or maybe slightly better on some games.
if the 580 you're looking at costs around the same, and you don't have the patience to see the benchmarks, get it. it's still a good card. personally, if it's no hurry, wait till the benchmarks come out.
So I should right now get the GTX 580?