On a vaguely related note (it is a small machine) I can recommend the Lenovo thinkcentre A61e USFF machines Ebuyer are peddling out for silly money at the moment.
The powerbrick is only rated at 120W or so, I'd imagine they are pretty efficient and seem really quiet when running. The CPU is the new BE low power range (45W from memory, the 2GHz Via is about 20, but at idle there will be a smaller difference) so overall this probably isn't significantly any worse on power consumption than Via based machines.
Dual core AMD, 1GB RAM, 160GB HDD, DVR-RW in a mini-itx like format for £195+VAT, pretty much zero expansion possibilities beyond what you can plug into USB and of course they are pre-installed with XP. But if you are looking for a small and quiet machine then certainly worth a look.