Thanks for the answers.
The fan is a cheap 12V DC computer case fan, like this one:
I've used PWM because I couldn't think of a cheap & easy way to output a controlled voltage varying from 0 to 12V using the Arduino, so I'm running a 12V line where the fan and the transistor are wired and I'm connecting pin 11 of the Arduino to the gate of the transistor, so I can have a 12V PWM signal. Here's the basic circuit:
+12V
|
.-.
| |
| | Fan
'-'
|
Pin 11 - PWM |
|/
o-------|
|>
|
|
|
|
===
GND
Thanks