A 550Ti is a little faster than a cheaper 6770 in general, but it's slower than the 6790 or GTX 460 which cost the same. So yeah, a 550ti isn't the best buy. Anyway, with few exceptions many modern games show a benefit with at least a third core. I clocked my old Brisbane at about 3.2Ghz and even then it was a little limiting for two 3850s at 1680x1050. It just depends on the games and settings, but yes I would say that in general your CPU would limit a 6770 or better card (and in general the 550ti is slightly better).
the 6770 is the same as a 5770 and performs better than the 550 ti generally