Well, as a data point for super old PC's, I kept around a Pentium 200Mhz running linux as my web development server right up until last year. It's amazing how little computing power you really need for some things. The main reason I got rid of the Pentium wasn't for speed anyway, it was because it was HUGE and incredibly noisy.

Anyway, at idle it consumed 32 watts and was something like 40 watts at full load. The CD-ROM, hard drive, incredibly old an no doubt very inefficient power supply, ancient motherboard, and whatever else were no doubt the cause of the relatively high power consumption. Based on my old P200's power consumption, I would guess that a vintage 486DX-50Mhz with all vintage hardware (power supply, hd, and so on) would still pull at least 25 to 30 watts at idle, and probably up to 35 watts under load. In terms of eletricity consumed per CPU power, those old systems would be down near the bottom of the ranking.

A P3 Celeron based system at least has the advantage of using more modern chipsets and such while also being very inexpensive. I typically run my system at 166x4 (664Mhz) so I can attest to very little CPU power being needed for all your general computing functions such as web surfing, email, MS Word and Excel (having 512mb of ram helps) and even graphic manipulation when working with smaller file sizes. I usually only kick up to 2Ghz when working with full size pics out of my 6.3mp digital camera.