Jay,
Let's say you lived in some future society where all computer parts worked precisely to spec. In that case you would want 350w for this system. And that's only because ATI scares the hell out of you by saying hey dude get a monster psu if you use our card.
I have built systems using every card and config imagineable and have yet to find one that did not work fine with a generic 300w psu. But that's just one guy and you need to figure in the horror stories of the zillion story tellers out there too.
There is an issue here in that all psus over 350w are just wild guesses by the manufacturer. A psu rated at 500W just might be better than one rated at 420W, but even that 20% difference is a guess. This is also true for memory (400mhz vs 333mhz) but is more pronouced with psus. All manufacturers make a rating guess, trading off the value of presale perception with the risk of post-sale support.
So what do you do?
Well the easiest solution is to buy from the one vendor who actually makes top shelf psu products, power and cooling. The problems with that is you pay through the nose.
The next option is to order yourself a generic. No, that is just dumb. But a fun bet.
The most economically rational option is to order from the ecs of psus, raidmax/powmax (same outfit). They make cheap psus but are a cut above generic and are likely to work just fine.
