The number of monitors you run has nothing to do with the power supply... I've been running dual monitors for years, although sometimes I wish I was running 3 or 4... My computer right now has a 500W power supply, which I bought to power an 8600GTS video card and extra RAM, but I have also run dual monitors with a standard 150W power supply... It isn't the monitors that you have to count, it's the video cards, harddrives, optical drives (CD, DVD, etc), RAM, CPU, fans and anything else that plugs directly into the power supply itself...
For running dual video cards, I would recommend at least a 1000W PSU, but it depends on the requirement for the cards you actually get... The video cards tell you in the specs how much power they need... If you are getting 2 of the same card, then double it... Then figure in for your fans and everything else, just add all the watts together... Then for safety, multiply the total by 1.1 or 1.2 (add 10-20%) and buy that size or larger... If you go smaller, you can count on replacing the power supply everytime your power company farts...
Just my past experience (and EET education)...
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