Wattage is not important. Amperage is, specfically the +12 Volt rail. If you have plenty of amps on the +12 volt rail, you will also have plenty of watts automatically. I'd say go for a minimum of 38 Amps on the +12 volt. That should power any dual CPU you want, and should power an SLI or Crossfire rig no problem.
Of utmost importance to me personally when building a box is noise. If you need help on quiet PSU's with good amperage, look at a company by the name of Seasonic. They may be a little more exotic than some of the more mainstream PSU's, but they deliver the goods with no sound at all.
If RAW power is what you want, look no further than Silverstone. Expect to pay a mint for Silverstone PSU's though. They are what I would consider industrial PSU's. These will not be as quiet as the Seasonics though.
-S
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