Power for 2x geforce 580

Welcome to the forum

If you're putting together a computer, you should start a thread by answering these questions: http://www.tomshardware.com/forum/261222-31-build-advic...
We'll be able to give you information that's a lot more suited to your needs.

The power supply won't be powering the monitor. But for the computer, an 850W from Antec, Corsair, Seasonic, or XFX should do fine. Some people might say a 950W. If you go for a cheaper brand, it may blow up in your computer and kill everything--just so you know. Here's a helpful resource: http://www.slizone.com/object/slizone_build_psu.html

What do you plan on doing with 2x GTX 580's? F@H? I'd save $300 and get two 6970's since the 2GB frame buffer helps at the large resolutions where you could actually make use of this much graphics horsepower. If you're only gaming at 1920x1080--a single GTX 580 can do everything you'll want. Of course, they also have the 3GB GTX 580's...