Well the MSI Titan system I was talking about arrived yesterday, it's certainly very compact, but with all the 'bits' you could want.
I have installed Ubuntu Server 7.10 on it, it installed without a hitch, not a single problem except that I somehow mis-typed my password during installation and couldn't log in when the system booted. Oops! However running 'rescue system' from the CD allowed me to run as root and modify the password file.
It runs from a little 60 watt external power supply which is just luke warm after being on overnight so around 20 watts or so consumption seems about right.
It's not incredibly quiet, the fan is quite noticeable with it sat on my desk, that's not an issue for me as it's going to be in the garage but it might be for some people. I've got an external disk drive sat on top of it at the moment so it may be a bit warmer than it should, the fan is CPU controlled.
I've already got the external USB drive working with it, I haven't dared try and re-format it yet though (any answers on that front?).
All in all I'm very pleased with it, exactly what I want at not too bad a price.