I usually run two GPU clients at a time. Running three plus the CPU will trip the protection circuitry in my Corsair AX1200 power supply after a while. It has also tripped the breaker once.
Wow. And I thought I used a lot of juice with a Q6600 @ 3.6, and two GTX460 1GB OC cards at 820-840Mhz.
I was powering all of that off of a single Antec EarthWatts 650W PSU, and it handled it 24/7 for a month without a problem.
3 GTX580s is what, like 700-750W of power all by themselves?