Well, I don't think I really need an enclosure with RAID 5 if the server already is RAID 5, do I? The data is usually between 1-1.5 TB and I want to take the backups off-site. I figure 2 x 750 GB or 3 x 500 GB drives would do. I already own Backup Exec, so I don't see any need to switch although I never seriously considered using Backup NT.
I will keep these suggestions in mind... maybe I will need it to scale for the future. FreeNAS mentions it's alpha/beta, although it seems like a good solution too.