There is no official release date for the 6xx cards, though after spending hours searching on the net, alot indicates that they're being released in 2/3 grand releases, Q3 for the high-end cards, Q2 for the mid-end cards and Q1 for the low-end cards. As for the 560's successor, 660, i'd say they'll be released Q1. As for myself, im waiting for the new 660 to release, instead of getting 2 560's now and then trashing them when the 660's release.
Edit : Wiki says around 200-350$, though unreliable i'd price them about the same. Yes, they are definately worth waiting for, as for the 660, its rumored to be equivalent to the 580.