No. DEFINITELY NOT!! The performance gain is negligible, at best. You pretty much have to take the inside of your 17" MBP completely apart, heat the old GPU enough that the connections separate, and resolder the new one in place. Not worth it to me even if someone gave me the 6770(as long as it's the stock Apple one) for free!
If I'm swapping out a GPU, I wouldn't replace it with a stock Apple GPU option. As long as you use a GPU from the same family(AMD HD6xxx), it should work as long as the power consumptions are equal. You can pick up a faster GPU chip with twice the memory for $50 if you're willing to put the work in it.