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5870 is probably the max it could support, unless the PSU is powering both systems at once?
Clearly he is looking for a 2nd computer and wants help to choose which Video card to take.... I hope.
People are WAY too conservative when it comes to PSU requirements and Gfx cards....
I assumed my Corsair HX520 would handle my 4870X2, but I got hardlocks until I bought a new PSU.