i run 3 cards on a 750w psu ( 700-725w at the wall )
That's easy...some cards only pull 15watts!
The poster I was referring to was running 3 7950s (underclocked).
If he was running these under load (scrypt mining/over clocked) he would not be able to run them with a 850psu. Additionally he would be burning a lot of wasted power because of the efficiency drop.