Be aware tha RAID is not a substitue for a regular verified backup.
Difficult one this, these days the only way you can sensibly backup the scale of data we all seem to generate is to other hard drives.
Even in work, although we backup our exchange email and most important documents to tape with an offsite rotation, the bulk of the data we generate is just too massive to backup to anything other than more hard drives (lots of graphics and video stuff).
So we have massive storage servers that are each multiply redundant running RAID6 that then mirror to an identical storage server. So at any time our data should be in three places, the originating machine, the primary backup server and the secondary mirror server.
It's a case of how far do you want to go?
E-I