The original poster's request for a small, quiet rackmount system with a fast CPU seems pretty difficult to achieve.
VIA C3 CPUs are good at what they do, but I doubt they have enough CPU power to be practical.
You could look into the Zalman Reserator (sp?) - that could probably cool a rackmount CPU without any fans, but it's huge, and quite expensive, $200+
How do you feel about water-cooling?