I was telling Narelle about Linutop computers and how they were pretty cool, but too expensive for they are when you can get a big computer for 350EUR, and that Linutop's pretty much force you to have a server of some kind anyway. She came up with an interesting idea: with an external USB disc, you actually have a full computer.
I'll track my research on the topic of 'superlight, super-silent' computers here.
Linutop 1 considered too small (256Mb not really enough for Firefox (what about FF3?)) Linutop 2: 333EUR incl VAT
An alternative to the USB enclosure would be to actually get a dedicated disc server such as this Quad-disc NAS. What's interesting here is the ability to scale: get one NAS with 500GB then several Linutops working off the same storage.
EDIT 13SEP2008: An alternative would be a FitPC which has an internal disc for a similar price range.