One elegant solution would be to make a thermally-controlled setup with a flat thermistor next to the side of the CPU core or (better yet) epoxied under the center of the core. Use that temp (should be in the 30-45C range) to adjust the voltage applied to the fan on the radiator (use an LM317T). You would have a knob on the front of the case still (the potentiometer) that would allow you to dial in how much the fan speed varies with temperature and also this would let you override the temp control. The circuit isn't too complicated:
http://www.tweaktech.com/complab/vfcu.htm
That will give you quiet when the system is idle, airflow when the system is under load, and a fun weekend project
