While I agree with Emperus's numbers, you need to add a bit more so that the PSU isn't running at 100% while your gaming. Not only would it be running very inefficiently, but you'll be causing it to work harder then it should. Getting what you need + 20% or so is fine. 550W *1.2 = 660W, so perhaps a nice 700W? 750W if you want to OC the GTX460s.
i agree.. And that's the reason i suggested a 750W PSU.. My point was to keep the overkill to as minimum as possible..