I use the pcmods.com rheobus - if you're just doing the one fan then that's of course overkill. I've seen something like this around a few places:
http://www.sidewindercomputers.com/rheosformc.html
zalman also has one, but it uses 3 pin connectors. Most of the rheostats you'll see on sites are 5 watts, you can't go too wrong.