the GTX 550ti is the winner here
comparison you did here is not fair...it should be something like 6770 vs 550ti
BTW you didn't mentioned your gaming requirements
at what resoultion ?
do you want AA ?
what games are you planning to play
If i was at your place i would have dumped 550ti and looked up for HD6850...
you'll notice day and night difference
PS: make sure you have a powerful PSU