Profiling the Power Usage of a Desktop PC

For small power measurements (Standby power, say in the <10W range), adding a light bulb in parallel should improve measurement accuracy.

If you can afford the higher prices, go with notebook disk drives to save power. Or use on of the recent 'Green power' offerings by some manufacturers. In general, keeping the disk spinning is where most of the power goes for an idle disk. And notebook disks are spinning slower and have less mass.

As someone else said, newer 80+ power supplies are more efficient and all currently available power supplies should have power factor compensation. But the latter has basically nothing to do with efficiency, only with the type of load. Ohmic loads have a power factor of 1.0 and that's generally desirable as far as the power network itself is concerned.

My current server uses about 50W idle running Linux.
That's with a 2GHz Athlon64 (idling at 1GHz and reduced voltage thanks to cpufreq), two 500GB 7200RPM 3.5" hard drives and an old PCI graphics card.

Next time I'll go for integrated graphics, I read that AMD chipsets have an especially good power/performance ratio.
And when I upgrade the raid disks I'll go for 'green' hard disks (or maybe notebook disks).