There really isn't a required fan speed.
Either the fan blows enough air that it keeps the processor cool, or it doesn't. The only way to tell really is to put it on and watch it for a bit to see how it does.

The fan on my freaking Volcano6 for y P3 just died so I tie-wrapped a big 80mm case fan to the heatsink, sure it spins slower but it works just as well because of the amount of air it pushes. =)

A+ Certified PC Tech Guy/Seitentaisei