I have 4 x 500Gb 7200 Seagate ST9500530NS SATA discs sitting here which I would like to use in some sort of NAS enclosure, ideally at least two drives in the same enclosure for RAID1. Would happily get a rackmount enclosure (Mrs VXL V6 is most impressed having a 19" rack in the house, honest!) to take all four if I could find one cheap!
I use an Infrant ReadyNAS NV+ enclosure with the drives (4x 2 TB) configured in X-RAID (Expandable RAID 5) which gives me 6TB of usable storage with a fair amount of redundancy.
Two drives would need to fail simultaneously for data loss to occur, the drives are hot swappable, and the enclosure can also be an FTP Server, Web Server, Print Server, Torrent Client, Media Streamer (DLNA compliant), yada, yada, yada.....all in an enclosure not much bigger than the 4 drives themselves.
Power consumption is only 58W when the box is being thrashed to the limit, but normally sits around 45W in normal use so no concerns over how much it cost to leave on 24/7.