If you're still thinking, just wait.
I think a 7770 is going to quickly disappoint you. A 7850 is the lowest I'd go in ATi land these days.
And an HD7850 demands two 6-pin PCI-E plugs; the HD7770 requires a mere one. (And that is quite aside from the $100USD difference in price.)
You are thinking of the 256-bit memory bus, which makes sense for 1920x1080 @ High or Ultra - that, at least to me, defines the beginning of the high end.
However, 1920x1080 is the cap for my display, and the odds of my getting one with a taller resolution any time soon are nearly nil (as are those of my adding a display).
Therefore, my going that spendy is practically pointless.
I'm thinking more like 1680x1050 @ High (sensible, given a 23" display - a single one), and therefore the HD7770 or equivalent (three months time). If anything, I'm thinking HD8000 will launch (paper launch) *after* Crysis 3, not before.