I can't honestly imagine that the 7770 is on par with the 6850- I know the clock speeds are faster, but it's a battle of a 128-bit card against a 256-bit card with only what, 225mhz core and 125mhz memory clock to make up the difference? That isn't even mentioning that the 6850 is known to OC fairly well from both my own experience and from what I've seen in benchmarks. I would wager the 7770 might be able to keep pace for lower-resolution scenarios and 2D rendering, but after that I think it's going to fall back.
I could always be wrong, but if I am, that makes the 7770 a heck of a steal considering the price-point of the 6850 after the same time on the market.