The 7950 beats 550 TI SLI in performance while being more energy efficient, far less stutter-ridden (maybe you wouldn't notice it on those 550 Ti cards, but maybe you would), and all-around better in most other ways too. Heck, if you care about PhysX, you could even run the 550 Ti as a PhysX processing unit for the 7950 and get the best of both worlds that way.
...With the 550ti SLI you would also only have 1GB VRAM, that's a good price for that model of 7950 too and comes with the 3 games deal so in theory works out cheaper than the 550ti sli if you were intending on buying the games
Two 550 ti's ? vs 7950 lol i would not even think about it get the 7950 550 Ti no offense to anybody that owns one are pretty weak bud get that 7950 and enjoy gaming like a pimp. 60+FPS in most games @1920 x 1080 FTW!