I run an oc'd I7-930 ( 3.8 ) , a GTX 570 SC, 1 HD , 1 OD, half a dozen fans and a water pump on a 750 watt Antec.
My little Killawatt meter shows no more than 410 watts from the wall when gaming, that's around a 350 watt load on the psu, wouldn't hesitate to add a second card.