Athlon 64 Quiet'N'Cool
X-bit labs - Articles - AMD Athlon 64 3200 CPU Review (page 4). Very cool point about the Athlon is that with the right Athlon 64 CPU (look at the serial number on the chip, it should end in "AP" and if you have the right motherboard BIOS, it should have a quiet'n'cool entry in it, and if you download the right processor driver from "AMD":http://www.amd.com/us-en/Processors/TechnicalResources/0,,30_182_871.
Then you can install and XP will dynamically change the speed of the internal CPU from 800MHz all the way up to its rated maximum. It will also change the voltage from 1.3 to 1.5.
This change heat dissipation from a high of 90 watts to a low of 35!