A .36 amp 120mm fan is probably spinning in the 2000-2500 RPM neighborhood, which I would think would be enough but who knows.

Let's say you buy this fan or a simular one:

http://www.newegg.com/Product/Produc...82E16835119136
It draws .6 amps.

First let's find the resistance of the fan by using ohm's law R = V/I

12/.6 = 20 ohms.

Now let's assume you went with the 100 ohm potentiometer like this one:

http://www.amazon.com/100-Linear-Tap...+potentiometer
Now we can find the total resistance of the circuit with the potentiometer maxed out.

Total Resistnace = R1 + R2

In our case

100 ohms + 20 ohms = 120 ohms

So the total current for the circuit would be figured

I = V/R

12/120 = .1 amps

Now we can find the voltage drop across the 2, we need to make sure we can turn the voltage way down if you needed to in order to manage the speed of the fan. Using's Ohm's Law again.

V=IR

. 1 x 100 = 10 volts

.1 x 20 = 2 volts

I think 2 volts is plenty low enough.

A lot of guys use 6 volt power supply with a 25 ohm resistor.

Using the same fan

6/.6 = 10 ohms

25 ohms + 10 ohms = 35 ohms (our total resistance)

Total current

6/35 = .171 amps

Voltage Across the reistors

.171 x 25 ohms = 4.29 volts

.171 x 10 ohms = 1.71 volts (at the fan)

Actually now that I think about it here is a high speed 120mm fan that uses 4.8 amps:

http://www.newegg.com/Product/Produc...82E16835706015
You'd still be able to get the voltage down to .3 volts with it.

Hopefully I did all that correctly, if I didn't maybe one of the egineers could please correct me.