Brand new tf 7950s were that price about a week ago. Other model 7950s are roughly that price currently. Personally I wouldn't buy used at the same price as new.
At stock speeds sure it runs. Problem will be when you overclock them.
EDIT: made the calculation at PSUEngine, given me 661W at Full Load with your specs(Overclocked+RAID(0+1)+5 Usb devices) and the three Radeons.