If you look at pure specs the 7850 is ahead of the 650 Ti Boost (http://gpuboss.com/gpus/Radeon-HD-7850-vs-GeForce-GTX-650-Ti-Boost),
and Physx is used in less than a handful of game titles also considering that AMD has Mantle (which is gonna be quite a game changer pun intended
I'd say go for the 7850.
Or the new R9 270X (a re-branded 7870, basically the same price as a 7850)
You will be much happier with your investment in the future.