physx is nice but not a game changer as only so many games utilise it. another thing to think about is the release of mantle for amd cards the 7800 series and up will support it and is meant to give us a good increase in performance for gaming
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-6...), 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.