I've gone for my old Shuttle SN45G, running Ubuntu 9.04 "Jaunty Jackalope", with an internal 250GB desktop disk and (at the moment) a Western Digital MyBook 1TB disk using USB2. I'm using LVM on the external disk so that I can (hopefully easily) add more storage later.

Backup will probably be achieved with another external disk and rdiff-backup.

I'm considering replacing the 250GB disk with a CF/IDE adapter (because it doesn't seem possible to get PATA SSD disks).

Even on permanently, it draws 16W for the Shuttle, plus whatever the external disk needs (usually spun down).