Temp sensor to turn on a circuit

[shaiss]

I'm building a 3d printer with dual extruders and two separate fans.  Both are run to the same 12v pmw pins on the ramps Arduino board.  However, I'd like to wire a "thermal switch" inline with the 2nd fan.  This switch would turn on when the thermistor on the 2nd hotend exceeds ~100°c roughly. 

Questions:
Could I use the thermistor that's used on the hotend as the temp sensor?
What logic ICS or something else would I need to buy to turn on above a certain temp?
How would the min temp be set? Resistors/capacitors?
Any rough sketch of a circuit would help and be greatly appreciated. 

I can attempt to mock something up in eaglecad or another ecad tool if that helps.

[alexanderbrevig]

You can use an opamp as a comparator. Use a voltage divider on both inputs. One is fixed a the threshold, the other varies with the thermistor. Should be plenty references on google

[Signal32]

you could use an analog pin on the arduino to read the voltage across the thermister and figure out the temperature in code.
Then use another pin on the arduino to turn on/off the fan.
No extra logic IC's.

As alexanderbrevig suggested, you could also do something like this:

[Ice-Tea]

Airpax.