Tiny development system with external power supply

I'm thinking about buying another computer to use pretty much solely for learning and experimenting with driver development and general programming. What I'm looking for is a tiny computer, which most importantly should be as quite as possible. Performance is not so much of an issue, and graphics not at all. Processor architecture is not so important either, although one of AMD's 64-bit processors would be fun.

Do you have any experience or recommendations on what to buy? I've been thinking about one of these tiny cases with external power supply and no fans, any opinions on those?

I just rebuilt 2 Dell Ultra Small Form Factor computers for work, they were very quiet (except when I blocked the airflow to one of them) and they were loaded with P4's, so if you get one with a Core 2 or a Celeron, or an X2 they should run cooler and be even quieter.