I have considered using an external drive for photo backup. My worry is that I won't remember to keep the backup up-to-date. Even if I do there will probably be quite a lag between new photos going on the PC and them getting backed up. For photos I am also considering using DVDs, but I don't know if I would be disciplined enough to check the backups regularly.
For me, backup needs to be totally automatic and, for the photos at least, pretty quick after they get put on the PC becuase I don't want to lose any.
Remember that drive failure is not the only way to lose data. One of the most common reasons to reach for the backup is that you accidently deleted it.
In my case all my data is on a "server" in the living room at the end of the coffee table, I'd guess its most at risk of a stray coffee and I don't think RAID will help with that.