The GTX260 is plenty fast enough run all the physx for any game out there. The question is what games are you playing that use physx? In a game like Arkam the 660ti will run on the highest settings with physx with no problem. Off loading the extra over head of physx may get you an extra 5 or 10 FPS but if your already at like 70-80 then its not really much use.
In my case if I turn off sli and use 1 470 for games and 1 470 for physx the games looks and plays exactly the same.
In short the gtx260 will work fine, I think you should try and see if you get any noticable fps increase.
I only have like 3 games that support physx, so i dont really see any value in it.