While working on Bill's Athlon 64, I discovered there are some things you can do for hot Athlon XPs. In the same "SilentPC":http://forums.silentpcreview.com/viewtopic.php?p=64530 piece, but there are some utilities that allow an Athlon XP to actually halt when idle. Apparently Windows XP doesn't do this, so you need a special little utility to do it and hopefully reduce power since most of the time the system is waiting around for a keyboard input or something.
You need a couple of programs. One called vcool and another called coolbits apparently.