PSU model please. while you don't need anywhere near 450 W for a 6770 system the sad facts of life are some 450W PSUs put out less than 250W.
The rest of your system specs would help as well. Also use gpuz and enable the log and play a game so see what its usage % is and check for temp problems
I have no clue to be honest, but if that's the case, would it really reduce performance that much? If my power supply was THAT low would seem like it would just crash.