HD 6950 and/or 560 TI is about the level where "real gaming starts" for a lot of people.
I can't guarantee you will get your 40 FPS with the 550 TI, no.
You can look it up yourself, though. Just type "i3-2120 + 550 TI FPS world of warcraft" in Google and somebody probably tested that before and let everybody know.
You can do the same thing with pretty much any combination of stuff.