A backup on a second raid disk is great in all intents and purposes but I'd say for that extra bit of security a backup on a completely different machine at a different location maybe even with a different provider would be a good thing.

I keep occasionally badgering one of my customers I support about their backups. They have a server with RAID disks which they backup but the backup never goes off site because they only have one backup drive. No matter how much I tell them that they need AT LEAST one other backup drive so they can alternate the drives and have an off-site backup it just falls on deaf ears because they won't spend the money on a bog standard USB hard drive (they don't have much data, easily under 160GB!).

Rob