Hi,
I have built this circuit which works like a dream as they automatically alter the speed according to the ambient temperature and will switch off when not needed. The circuit will drive 4 or 5, 3 inch 12V fans drawing about 200mA each.
**broken link removed**
Cheers
Andrew
PS: The circuit does not use thermistors but diodes which rely on ambient rather than actual temperature. By placing the sensing diodes close to the equpment you can sense the temperature in that manner.