the system i am typing on stays on all the time, unless i go on vacation, them my dad just pulls the plug without telling me....:| (win2k does NOT like that.....) The system I am typing on is actually in a drawer under my bed, with no case. It uses about 60 watts in its idle state with the monitor off. The harddrive has about 6 towels over it which really drowns out the niose it makes. I have a fan rigged so that air flows through to cool it. As far as the processor, I took a heatsink and a 12volt fan and hooked the fan up to a 4.5 volt power supply, so it is very quite. I can do this without overheating because I am using a K6-3 400, which creates little heat. The power supply is also very quite.
My power system, duron 650 and radeon LE, stays off at night most of the time. It is in the living room, so it wouldn't bother me when I sleep, but there is no point in leaving it on.