I think 3SLI would really push that 850W PSU to its edge, those cards can be somewhat power-hungry. It should work, but the scaling drops off a decent amount from 2cards to 3cards and even more importantly really depends on the game/etc as well. I don't think it would be worth it to pay that much for a 460 to do dedicated physx, may as well get a 9-series card for cheap to do it if it really mattered (not too many games have physx support from what I can tell).
http://en.wikipedia.org/wiki/PhysX
Yeah, not a very staggering list.