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).

A pair of NSLU2s with big USB drives was my first thought, too. The NSLU2 is small and silent, and you can pop them open and remove the intentional underclocking by clipping a resistor. (http://www.nslu2-linux.org/ has details.)