I can't give you a circuit schematic of how to do it, but this semi-broken link
to a discussion on arstechnica shows that it's possible to set a minimum speed for the fan.
Found it. Go to http://people.freenet.de/s.urfer/fan_control.htm
. It does everything
you need, including having a min. voltage and a max-voltage startup for those fans that won't start at low voltage. It's complicated though. I think parts of the circuit can be grafted onto the simpler one that Rusty linked up. When I have a chance to experiment, I'll post my findings.