the recent driver update has closed the gap between the 7950 and 660ti quite a bit. now the question is; are you willing to pay a bit more for a 5-8fps boost in some games?
summary; if you're fine with FXAA and willing to pay for it, no reason not to get the 660ti. heck, it'll make for a good dedicated PhysX card in the future when you upgrade you're GPU. if for some reason you prefer 4-8xMSAA, welp, the 7950's the way to go.