[GuyBFF]

Another way to make a reliable shutdown controller is with a DigiDoc 5 and a relay to one of the fan outputs. This will obviously cost more, but I like the DigiDoc 5 for other reasons.

Wire the relay to be normally closed and connect between the power good to the motherboard. This way you have a temperature setable shutdown.

I also use MBM5 and SHDN like bladerunner suggested, but if my CPU is running at 1700+ it will lock if it warms to much before shutting down, so I have a backup.

Another thing the DigiDoc can do is control fan speed to a preset if you use a LM317T, one resistor and a pot. Basically you can set a low speed, then when the DigiDoc kicks in it will increase voltage to the full 12V. Because the LM317T will see a higher signal on the ADJ it won't add any more power, and won't allow any sort of current drain. The reason I use a voltage regulator is they will supply current without dropping voltage like a rheostat or resistor at fan start (where DC fans can draw 5 times the current, droping voltage to a point where the fan won't sometimes start).