I have just a single Evga Gtx 560 TI 448 Ultra and want to get another but not sure if it will run on a 850w PSU.
Intel Core i7 975x @ 3.4Ghz
12GB Crucial Ballistix Sport 1600Mhz Ram
NZXT Switch 810 Case
GTX 560 TI 448 Ultra
GT 440 (physx card)
Asus Rampage III Formula Mobo
Thermaltake Touphpower Grand 850w PSU
Two 2TB Hdd's Raid 1
Windows 7 Pro 64 Bit
Two of those cards in sli require about 800 watts. If you had a Corsair, Antec, XFX, or other top tier powersupply I would say yes, with no problem. With the one you have, it should if the unit really puts out what Thermaltake says it does.