If they are ever hot I would just install a few fans that run full time. However, if you want to have fans come on at a specific temperature I would consider a thermistor circuit or maybe run with one of the many LM35 circuits out there. Note the circuits in the **broken link removed** Many could be easily adapted to run with a comparator circuit to turn fans on or off based on sensed temperatures.
Eric provided a link to some thermal switches that would do just fine. The link I provided for the LM35 data sheet has some circuits that could be used as a start. Then if you want more there is this circuit and also there is **broken link removed**
I like Eric's suggestion of using a thermal switch. Very simple and would work just fine.