It just so happens that both cards are on the same level and that makes them equal and what happens is that each card will play certian games better than the other one and the reverse is true as well. So while the 550Ti might be better at BF3 the 6770 might be better at Metro 2033 and it will go back and forth like that for all of the games. You will have to look at the games that you play and then look at the benchmarks of each card playing those games and choose the one that does the best in the majority of the games. It sounds like a lot of research to do and if you don't want to do that then you could just choose one.
The cpu that you have is a dual core running stock at 2.8 ghz so if need be the cpu can be clocked a little higher to alieveate any bottleneck if you happen to have one.