I set up FreeNAS on an Asus P5B (Basic) with a Pentium D 3ghz processor (may be overkill, but it is a spare machine). The CPU usually runs around 750mhz-1500mhz. I am aiming for a quiet machine so the only fan is on the CPU's Heatsink and the power supply. The hard drives and cpu heatsink remain cool to the touch, but the Northbridge's heatsink and the area around the power supply are very warm to the touch, bordering on hot.
I am not sure if too much can be done for the power supply, but for the North bridge, are there any settings I can change in the BIOS (or even FreeNAS) which will help that run cooler?