I was going to recomend NAS as well, but since that's out, I'd go DLT, much more expensive than DAT, but has much greater storage per tape (160/320), and much faster on the backups, and through my trials of dealing with tape backups, the DLT tapes are more reliable.

http://www.quantum.com/Products/TapeDrives/DLT/DLTV4/Index.aspx