Well, the 7770 is released and is overpriced compared to the 6850 which it competes with. As far as the 7850 and 7870 go, time will tell. I think the 6870 is currently the best at its price point.
Your resolution makes a big difference in the recommendation. If you are @1920x1080 then yeah, upgrading from a 5770 is very much needed. A 6870 would be a great upgrade. But I'd say that anything short of a 6870 isn't enough of an upgrade.
If you haven't already, read http://www.tomshardware.com/reviews/radeon-hd-7770-7750-benchmark,3135.html
Gives you an idea of where your current card is performing vs. the 6850, gtx560, and gtx560ti.