When designing it I had them include a 650 watt peak power supply. Recently the computer's been crashing when it was trying to do a lot of disk access. When I opened it up I noticed some ripple in some of the fans rotation, leading me to think the power supply was going.
In a burst of curiosity I plugged the server into my Watts-up meter so I could figure out what power supply it might actually need.
It's a single cpu 1.6 Ghz Athlon MP with 1 gig of ram and 7 drives in it.
You might think that the massive power supply was needed, but the peak reading I could see on boot, while it was spinning up all the drives was 270 watts, when idle it was hovering around 170 watts, which admittedly is 80 watts higher than my AMD64 desktops is still very far from needing a 550 watt power supply.
So why do they sell 600 watt power supplies? You might need one for a 4 way opteron, but that should be about it.