With RC5 running 24/7, and either stock AMD cooling or a FOP32-1 plus bay cooler and 80mm front case fan, my Athlon 900 ran at 50 to 53 degrees C at all times. If I removed the 80mm or bay cooler, the temperature wouldn't vary. If I removed the side panels, it varied slightly but not by much. I finally decided it had to be due to lack of airflow in the part of the case that the CPU sits in (my roommate's 900MHz Tbird runs at a steady 35C or so with no covers and nothing but the AMD cooling).
A few days ago I cut a hole in the side of the case and mounted a 92mm Sunon fan. Since then, the CPU temp hasn't gone above 45 degrees under full RC5 load, and it was averaging about 40, dropping as low as 35 if my room got cold.
When I woke up this morning it was at 45 degrees (the air in my room got a bit stuffy and warm). While reading this thread, I shut down RC5 and the CPU temp has stabilized at 31 degrees C. If my room were to cool down again like it was yesterday (wild temperature swings in here) then I'm sure the CPU could go down possibly as low as 25C.
Hehe, just noticed that my CPU is running at the same temp as the rest of the case. Woohoo! I've reached AMBIENT TEMPERATURE! (Well, ambient for the inside of the case anyway.)
Side blowholes rock. After mounting that I've been able to remove the bay cooler which kept pissing me off because it blew across my legs and was making a lot of noise without providing any cooling, and the front 80mm fan which again made a lot of noise but didn't pull in much air (the holes in cases for fans really are too small and too few in number to allow a fan to intake as much as it really can, and they cause it to make more noise while trying).