You are right modern PSU units have better efficency at lower load levels. I would expect to reduce my power consumption by 50% by upgrading the motherboard and CPU plus changing to a more efficent supply.

I would say since building this file server I have noted it generates lots of heat and this is a bad thing when you consider it sits idle most of the time – which re-enforces what you have stated about effency above.

There’s a number of fairly cheap (<$45 on Newegg) sub-400W *good quality* power supplies, the vast majority made by Seasonic and rebranded, with efficiency ratings well over 80%. Older PSUs, while a little more robust, tend to waste a great deal of power simply idling around even with a minimal load. Examples include the entire Seasonic line, Antec Earthwatts and Truepower models, and the Corsair HX### series. I’m sure there are others as well.

On another note, I’m not entirely certain as to your motherboard’s capacity or compatibility, but additional power savings could be had by switching the processor to a newer Allendale or Conroe-based CPU (even single-core chips are quite satisfactory). Either option would, in both Dual and Single core variants consume lower power than your current Pentium.