Split rail PSU's suck. Go with a powerful single 12V rail instead. I just purchased pcpowercooling 825watt psu with a kick ass 60amp 12v rail in preperation for the new cards. I dont have to worry about power being divided up among the rails.
Though it depends a bit upon the total amount of hardware you're running, I think the Enermax 620wt should do you fine, unless you decide to go SLI. From what I've read, a SLI setup would need closer to 800wt. Could be wrong about that. Will know more when the cards actually come out.