500w would be fine. I, personally, have 1 main machine that I upgrade incrementally (as most do). A PSU is one, if not the only, item that doesnt require "upgrading" if you knock it out on the first go around though. If you are in the same boat, you can essentially future proof your system as far as a PSU goes and get a 750w Corsair TX and be done with it, never having to worry if an upgrade will also need a better PSU later on down the road.