[andrewas]

You should use both, they are for different purposes. The case fan improves airflow through the case in general. It is possible to build a machine without one, but high-end processors put out enough heat that you really should have one.

The one on the P4 is directly cooling that chip, and no CPU today can operate without one, the heat generated is high enough to melt parts of the motherboard if it isnt dissipated quickly enough.