While it is true that PSUs have grown, and for some people having a bigger one is better, most modern computers are actually using less power than the infamous P4-era ones, and they are faster too.

If you want to try and be energy efficient I believe that the most useful thing is a watt-o-meter or something like that, so that you can actually check whether that huge 500W PSU is being used, or it’s just there for marketing and the PC is using even less than an older PC that uses most of the juice its 200W PSU can give.

Another thing to consider are monitors: most recent (<5 years) laptopts I've measured use less than 30W, screen backlight included; it is hard to get the same performance out of a standalone standard (cheap) LCD monitor, not to speak of older CRTs.

On the other hand, I have an atom powered 10" netbook and it takes a bit more power that I'd like it to take, at some 20W (when not charging), about the same of other laptops.
That's the one I use as my main PC, for internet browsing (requires X), work (i.e. using vim to edit text files of various nature and often compiling them to some other format) and some media use/creation.
The same things could probably be done on a pre-P4 computer, it would be interesting to know whether it can remain in the same power range.

For home servers, I have a couple of old low-power (~5W) small boxes and I plan to change them with a single *Plug or something like that, so that I can have just one always on, while being able to do some work on it even when spamassassin is running (right now it tends to take every resource and slow the computer to a crawl, so that I have to use the other one for anything interactive).
I don't think that even an headless P100 with .
a compact flash for storage can use as little as 5W power.

I also think, OTOH, that this is a transition phase: right now computers have just started being build for efficiency more than power, so it is worth getting one of the former instead of the latter, but in a few years it will probably be worth again using an old computer as long as it works, unless there is going to be another drastical reduction (e.g. from moving to a different arch).

Sometimes instead of desktop PC I use Fujitsu-Siemens Amilo Pro laptop (2005) which uses 30W if it doesn’t charge the battery.

My late 2009 triple core desktop with integrated graphics uses 120W with display on and no CPU load – more than all above computers together.

I really like low power requirements of Atom netbooks. Unfortunately I forgot to check real power consumption when I had one for a few days.

SheevaPlug and GuruPlug looks fun too, but they are not available in my country and shipping from US or UK is to expensive.

There is also – http://www.raspberrypi.org/ – 25$ pendrive sized pc project.

I thing it’s better to do something useful with old pc rather than throw it away (recycle).

There needs to be way more legislation passed that will stop Americans and other countries from shipping this stuff out too. If we can find ways to make these electronics, then we we can find ways to recycle them.

Older machines used to work fine with smaller power supplies, but these days even 500-600w psus don’t make the cut for many.

Netbooks are efficient, atom processors or arm processors are efficient, but most people still go for dual, quad or eight core machines, and they definitely aren’t efficient.

2. General Statement. Intended to invoke thought about carbon immitions released by running many old computers.