Thanks for the input. The silentpcreview site helped. Based on that, I'm looking at these two drives:

Samsung Spinpoint T Series HD400LJ 400GB, 7,200 RPM
Western Digital Caviar SE16 WD5000KS 500GB, 7,200 RPM

Both are SATA, so I could do one of these and get an enclosure that has a SATA and USB connector. I'll use the USB for now, and switch to e-SATA later if I get a connector for that in a machine down the road. Both are supposed to be very quiet.