adding a fan to an enclosed cabinet

This is like Deja-vu. I have a cabinet similar to Sonofnorway, purchased the variable dc power supply from Radio Shack like Chadnliz's friend, and use the exact fan pictured in the link by Mijknarf. The only thing I can add is whether you blow in our pull out, you are going to have about 5 times the dust you normally do so clean things with a soft brush attachment on a small vac regularly.

If you're interested, the fan I used is a Papst/EBM 8830N. I got it online from Newark InOne. Certainly not the cheapest, but it was the quietest I could find.

Go battery operated. Minimizes any potential interferences, usually quiter and resonable cost.

Sounds like many of us have gone through a similar exercise. A few years ago I had a cabinet custom made to house (most of) my equipment. The rear wall of the cabinet is 3" closer to the front than the top and both sides. This allowed me plenty of room for all the cables to be run behind the cabinet and out of sight. Heat however, was still a problem. I first mounted a small fan against the back wall, but it was too noisy even though it was the quietest I could find at the time. I ended up buying a cheap flexable cloths-dryer exhaust hose that I attached to the rear of the cabinet where I made the whole for the fan, and the other end to the fan which sits on the floor behind the cabinet. The result; very quiet operation and a great deal of air movement.
Hope this helps! Ken

Kgproperties, very cool implementation of a ventilation system. Depending on one's situation, the fan could actually be mounted in a basement, if the hose can be run through the wall and/or floor. By placing the cabinet openings at the bottom, sizing them appropriately for the air flow, and mounting the hose at the top of the cabinet, a bottom to top flow pattern should exhaust warmer air and allow an intake of cooler air.