Battlefield 3 multiplayer needs a quad core. Is your triple core unlocked? I run BF3 on mine with a triple core (mobo won't allow an unlock ) and sometimes it gets really choppy in 64 player team death match. With a quad core, and if you're running it at 720p you should be able to run it a high ish easily, with my GTS 450 DDR3 I can run it at medium high at 720p in campaign with 30fps as the lowest FPS I get near enough. 30fps is what the consoles run at so its far from choppy. Anything higher than 720p and you're asking a bit much. At 1680x1050 you can get medium high with more settings at medium with about 30fps average. My monitor doesn't go any higher so I can't tell you what it's like at 1080p.
At 720p, the GTX550Ti should be able to playable frame rates (Most likely 25-30) when all is set to 'High'. I've got a GTS450, and I had no issues running the game at 'High' settings at 720p. Although, for smoother gameplay, I'd recommend medium settings, with no AA.