hey gosu,
I ::think:: the ohms determine the ammount of resistance you can add to the line, thus lowering the speed of your fan. A rheostat will allow you to vary the resistance, but if you get an 8 ohm rheostat... I fear you might not be able to slow it down as much as you would like to.
Now, I am not positive, so hopefully someone else will come along and confirm this, but I think you'd want a higher ohm rated rheostat so that you will have a wider range of fan speed.
good luck!