Well, the following are your components maximum power requirements. Your CPU will use 120 or 140W depending on which stepping it has, will use the 140 figure, your card will use 150W. Then say 100W for all the fans, harddrives and RAM. So with one card you use 390, with two: 560W, that is your absolute maximum load, which it isn't very likely to reach very often. However a 650W PSU is what I would personally want to use.