Power Supply – Does It Draw Only as Much Power as Needed?

I recently built a small mini-itx Intel ATOM-based Ubuntu home server. The case I choose is a small case but only a full size ATX power supply would fit it. I choose a mid-grade OCZ 500 watt modular power supply for it and it works great. I could not find a modular ATX power supply out there that was less than 450W.

So my question is, does my 500W power supply draw 500W just because thats what it is? Or does the power supply only draw as much power as is needed to power the computer components?

One mini-itx ATOM board + two SATA HDDs = less than 100W I figure. My goal was to build a low cost, low power consumption server, so hopefully the 500W power supply isn't drawing 500W.