Quoting directly from Dell E510/5150 Service Manual:
"SpeedStep (On=default) Allows the BIOS to modify the processor fan and speed based on processor temperature readings.
NOTE: Intel SpeedStep® is not supported by all processors; therefore, this option will appear only when a supported processor is installed."
Actually my BIOS says the default is OFF. Odd thing is, in my experience with 3 of these PCs, whether OFF or ON, the CPU will idle at 14x and increase to 15x Multiplier under load. Also, only one of them would vary the fan speed; the others just stay at low-speed, almost dead-quiet, no matter what the CPU load.