i have a truepower 430W PSU and it has a fan monitor cable coming from it if i plug the cable into a rheostat will it slow the fan down or damage it, or would it be pointless?
Most likely, it would do nothing. Consider the fact that if you were to disconnect the cable from your motherboard, the fan still runs. It's obviously not dependant on the fan monitoring cable to get it's negative power source, so altering the resistance on that line should have no effect.
That wire outputs pulses to a mobo fan connector so that you can monitor the rpm of the fan in the psu. Putting a rheostat on it will do nothing other than give the circuitry delivering the pulses a load, and if that load is too small of a resistance you could burn out that circuitry.
