These guy's are the real deal, I am very young and very limited in knowledge, what I am trying to say is, dont go running off with my idea thinking it is the answer, because i might have it wrong! Anyway, there are some computer fans that 2 or 3 settings for speed, the speed of these fans is determined by which of the control wires is pulled high, so you could use that. Or another way
Quad comparator outputs to resistor network, resistor network connects to voltage regulator pin, voltage from regulator determined by the resistor, so you set comparators for different temps, so as temp rises more voltage to fan, this should give you a slow to fast fan with little effort. But I am not to be taken on my word, I sometimes see solutions that are not really there.
By the way have a very good look around for fans, i have two that run really fast at only 18mA!!!
yes 18mA not 180mA!!