Darn,

Reading through this forum I thought Id be able to provide some insight but Ernie beat me to it. We use cooling stats that we buy from a local HVAC company and tie them in to the fan... set them at 85 degrees and were done.

This may seem like overkill but we often use a Nutone LS-80 model bathroom fan and duct it outside (lots of custom labor this way. The best part of this fan is that it runs at 0.8 sones (very quiet). Most of the AC 4" fans are screamers.

You also need to make provisions for air intake. We usually drill holes below in the toe kick to bring room air into the cabinet or change doors to screened material. It is very important to keep your equipmnet cool... we always make cooling a part of any proposal we do.