Intel Atom (Score 3, Informative)

I wanted the same type of box a year ago, and settled on Intel's D945GCLF2 board, which has a dual core Atom, onboard video, one memory slot, and two SATA plugs. Adding a 2Gb stick, a 500Gb Seagate drive, a generic CD-RW, and a case to put it all in ran just over $300. Runs Ubuntu 8.10 like a champ, and draws 35 watts when spinning the disk. To quiet it down, I replaced the stock northbridge fan with a Zalman passive cooler (instructions here).