Which potentiometer to use?
I found this which might be helpful:

http://img.techpowerup.org/100617/fancontroller.png


You might need to adjust Q3 based on your stated power supply.

EDIT:  I should add the source of that:

http://www.techpowerup.com/forums/showthread.php?t=124633

That discussion also talk about using just a trimpot vs PWM.