Here's a Seagate 40 G (you said 20-30G was more than enough)...
And I just bought a 2.5" USB
enclosure on eBay for $10.
Stick those two together, format, and you're ready to go. I have an older one myself and just gave one to a friend. For all intents and purposes, they are silent. Also, the require no external power supply.
edit: Just make sure the enclosure supports USB 2.0 and that the drive is the same interface as the enclosure (PATA/PATA or SATA/SATA). Mine are both PATA, and I think those links are both for PATA as well.