tomahawk you should have at least 650 watt for your rig to be on safe side. Inadequate PSU in the most overlooked potential problem maker in new system builds. I have used both a 750 PSU ($130) and a 860 PSU ($240) both from PC Power & Cooling. I run the about the same options as you do except in the 750PSU rig, I have a 4870VGA and in the 860 Watt a 4870x2. No power problems whatsoever. Eliminate a number of potential problems and dig deeper to have adequate power.
@OP: The #of watts is irrelevant. What matters is the +12 amps and the build quality. Like I said before you can run a 9800GT on a Corsair 450VX. Please post EXACT PSU model.