I'm running 3x cards. PC is overclocked i7 to 4.02ghz 16gig ram. When I had 1x 7950 it was pulling approx 450w from the wall. When I had 1x 7950 and 1x 7850 it was pulling 590-620 from wall, that was on a Corsair gs600 psu, ran fine.
But i had to add another PSU to run to gpu's after adding another 7950 its now pulling 760-820 from wall depending on gpu voltage and whether im using usb port charging phone etc.

So mining at 1.4 Mh/s with the 7850 as main card, only running that at intensity 15 so I can still use pc and the 7950's are slightly under clocked and lower voltage, cause middle summer in Northern Australia this thing is cooking me in my room, and I dont like em running over 80c, the outside cards run 77c the center card 85c