Gday,
You might want to check out this site
http://www.silentpcreview.com
Currently I'm running an Athlon XP undervolted (to Vcore of 1.1V) and underclocked (to 600MHz) without a fan at all - just heatsink. At the moment here in Australia we're in 30 degree+ heat and it's running just fine at about 45 degrees celsius. Performance is much much better than my old epia m10000 too, and power consumption of the cpu is ~ 15 watts (using AMD's tool). The Epia's is ~ 10 watts.
Depending on what celeron and moterboard you have it should be undervoltable as the athlon is.
Hope everything goes well with it,
Dave