You need a MINIMUM of 400watts, so basically u want to go about 120w higher than this to be on the safe side, depending on what else u have in ur pc, and ur 12v rail requirements are 26A. Again depending on what else u have on ur pc.
Actually when i got the card in my rig, i had a psu with a crappy 12v rail, it crashed when the hoover was being used
and the games were slower.