660ti would be more than enough, but 650ti might be a tad slow as it's really a LOT slower card ... if it were me, if I was going to go with a separate card for physX I'd look for like a used GTX460, should be able to find a cheap one of those somewhere (a 768MB model would be perfect, as they should be really cheap & memory doesn't matter) & that'd be the perfect amount of power for what you're talking about.
Personally I like having a separate physX card for an SLI rig. Reason being, physX only runs on one GPU, but two cards in SLI have to run at the same speed, so what happens is you get a 'double hit' from the physX load ... one card gets slowed down due to the actual work of the physX, and then the other has to slow down an equal amount, just because. It makes for really sub-par SLI scaling on PhysX games.
Edited by brettjv - 2/2/13 at 8:40am