depends on the game. if its a more gpu intensive game than you wont notice much of a bottleneck.
but id just get the 6770. i have a 5770 (same card really) in a computer with a e6750 at 3.2 GHz and they seem like a perfect match. so getting a 6850 for your system would be stretching it. but you never really know until you try it or research it.