Hmm, I can't understand what you were trying to do with the 7805 in that circuit...theoretically your fan could only get 5V max..

LM324 has a 20mA Iout capability. Assuming you're using a similar spec fan to Frank, you could do a dirty hack to remove the need for a transistor/regulator. Use 1 of the OpAmps to do the detection as your circuit currently shows, and hook up the other 3 in parallel as a current driver to give you 60mA max driving current. (It's not the ideal way to do things but it should work.)