I went with two of these: Western Digital 2TB WD Green SATA III Intellipower (www.amazon.com)

They have nice "low power" modes, and seem to be fairly reliable, and are listed among the compatible drives on the Synology site. They aren't industrial grade, but two of them cost almost as much as a higher classification one, which unless you're planning on 24-hour, sustained reads/writes by multiple users for months at a time you probably don't need anything more expensive.