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
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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.
I had two 6970's on it but when I run the power supply calculator is says it requires at least 950w.
I don't see why two 560 ti 448's won't run when I had two HD 6970's. If I have to upgrade to a more powerful PSU I will but don't want to since I've only had the Thermaltake one for two months.