CentOS 5 Hardware :: Low Power Usage / Low Noise Hardware For 5.3 Recommendations?

I'm looking at various low power consumption system options for running CentOS 5 on. The goal is to get a system that draws 20 watts or less, is silent (but I'll settle for very quiet), and is fast enough to be a light duty server. The kinds of things I envisage for it are low volume mail, http, ftp, subversion, ssh, maybe samba, and perhaps asterisk if I get enthused enough. Wireless network support isn't something I'll need and realistically the system is going to spend most of its running life being idle. I don't foresee any particular need to run X on it as I imagine that if I ever need to configure something graphically I'll do so by forwarding X over ssh.

I'm leaning towards the Asus Eee Box B202 as it's readily available, seems to be in the ball park of what I want, and I see on the forums here that other people have had success running CentOS 5 on it. Still, I'd prefer a silent system (though I suspect silent systems may only be available at a premium). So I'm after war stories and recommendations. What other low power systems have you run? For the Eee Box B202 or other system what kinds of real world power usage figures have you seen with CentOS when idle and when under load? Did the system perform below your expectations? If you needed to change the hardware (say, replace or upgrade the hard drive), was that straightforward or was it a chore?