Nvidia nForce4 4X User Manual page 45

BIOS
Smart Fan example:
Only need to set the items in gray. The system will automatically
calculate intermediate temperature Temp LM and Temp MH and the
corresponding Duty LM and Duty MH.
How effective Smart CPU Fan is will depend on fan design. Most
fans have built-in thermistor and may self adjust its speed. Some
fans have very limited duty cycle range open to fan control.
For best results use the bundled USDM Software to tune your fan.
Page 4-19