If you go to the AMD web site and look at the "Builders Guide for desktop/ tower systems" it shows the AMD Athlon gets it power off the 12 v rail (eg - XP1800 1.533 GHz, 1.75V uses 6.87A or 82.44 watts). Not sure if this is true for all CPU's. This table shows by component typical Amps used and which rail - very helpful (unfortunately, I printed this guide out but didn't save the URL - think I got it from an Anandtech thread)
Also, it had a nice write up on air flow - shows that a two fan PSU has significant improvement on cpu cooling (draw from the bottome of the PSU - when installed next to the case exhast fan greatly improve air flow across the cpu HSF - as opposted to a PSU that has one fan and draws air from the back of the PSU).
my two cents worth