I have been playing with Speedfan to see just how little fan speed I can get away with whilst idling (or playing videos, browsing the net etc).
The answer seems to be a 20% fan speed with my Arctic Cooling F12 fan (1450rpm @ 12v). Importantly it starts at a very low speed, and at 20% (a mere 323 rpm) it is basically silent, and best of all it keeps my 95W CPU under 45C, above which Speedfan starts to increase the fan speed. The top and back fans are hard wired to 5v, the top fan is an F12, the back fan is a 1200rpm Slipstream.
I am only telling this to the SPCR readers because this is of interest to them. In reality this has made no difference at all to the noise floor of my PC due to my monitor outputting more noise than my PC and anyway my 2 case fans @ 5v are both running at about double the RMP of my CPU fan.
I hope that this is interesting to those people who have a truly silent PC and monitor (and live in a foam padded cell in a cave). Just how low you can go with "Speedfan" and have it cool your PC perfectly well, the answer is very low. I was suprised as well, I though that the 30% speed floor I had set for Speedfan was the lowest that would be able to work effectively, I was way off.
PS: Current readings CPU = 27/30C fan speed = 308 rpm, laptop HDD with barely any airflow 27C - my room it not warm ATM although I cant give you an actual temp reading.
Main PC, P180, CM Silent Pro 500M, i5 3570k @ 4.2Ghz, 8-GB @ 2,000MHz, 256 GB Samsung 830, 500-GB 7K500, MSI 660Ti Twin Frozr, PC is super quiet :o
Server, 6-TB RAID-5 array, + 2 x 2-TB backup drives, 380W Enermax Pro82+, 4x very quiet fans, positive pressure only, no exhaust fans
Living Room PC, 3500+, 2-GB RAM, HD501LJ