The Antec Fusion Black HTPC case has two of their TriCool fans, which provide a three-position switch for speed selection. They flow tremendous amounts of air, but even at their lowest setting they're audible, and they're not controllable by the motherboard.
Antec TriCool fan performance
RPM CFM dB 2000 79 30 1600 56 28 1200 39 25
In order for the motherboard to control a fan, it must have a three-pin connector. This provides voltage and ground, plus a sensor wire to sense the fan speed. Many fans provide only a four-pin Molex connector, which won't allow for any motherboard control.
This excellent fan comparison led me to order a pair of Scythe S-Flex fans. The fan features "Fluid Dynamic Bearings" licensed from Sony, and is available quite cheaply. The fans are available in three model designations, -D, -E, and -F, indicating low, medium, and high-speed, respectively.
Scythe S-Flex fan performance
Model RPM CFM dB SFF21D 800rpm 33.5CFM 8.7dBA SFF21E 1,200rpm 49.0CFM 20.1dBA SFF21F 1,600rpm 63.7CFM 28.0dBA
I bought the fastest of the three available with the optimistic hope that my Biostar mother board will do a reasonable job of regulating their actual speed as needed to control temperatures.
Update
I have since learned that the TF7050-M2 apparently cannot control the speed of case fans. The -F model is exactly as loud as you would expect a 30-ish-dB fan, which is far too loud for a home theater PC. So, I stopped by Central Computer and overpaid for the -D model (800rpm). It's still audible three feet away, but I'm sure I'll never hear it once it's in the entertainment center cabinet.
