PhysX matters about as much as a fly does to the altitude of a plane. That is to say, it really doesn't matter.
850W is more than enough for Quad SLI, and will be horribly inefficient, get a 600W PSU for triple SLI, or 700W for Quad.
TBH I'd start over, get a more efficient build with an ATI graphics card, the ATI range is cheaper and thus higher valued right now.
Efficient Gaming rig:
CPU :$ 50: Athlon II 240 X2
MB :$100: Asus M4A78T-E
GPU :$290: ATI Radeon HD 5850
RAM :$115: 4GB 1333Mhz DDR3
PSU :$ 50: Corsair 400W CX series
Case:$ 55: Antec Three Hundred
HS :$ 35: Gelid Tranquillo
HDD :$ 45: Samsung Spinpoint F3 500GB
Totl:$740:
Last edited by Vox; Jul 11, 2010 at 08:19 AM.