If you don't have multiple TB of data, go to an online, off-site backup provider.
As others have said, RAID is not backup -- at all. And if you have a storm or fire or theft at home, your backups don't do you any good, so you need an offsite copy. And an automated system is really the only way to get a reliable, off-site copy.
I recommend Crashplan. I've been using them for a while on a few different systems, and they're great. They retain multiple revisions of files, keep deleted files, etc. Prices are very reasonable, and the servers are speedy. For business use I recommend others, but for personal use I think they're the best out there. I've used DataDepositBox (now KineticD), iBackup, iDrive, Mozy, SpiderOak, and evaluated Carbonite, and I personally prefer Crashplan by a big margin.http://www.crashplan.com
If you don't want to pay for a subscription, you can still use their software for free and securely backup your data to friends/relatives computers, and vice versa. You can even seed the data using a portable drive so that the initial copy doesn't take forever.
Edit: They're also reasonably secure, and there's no hardware to maintain. So if you sign up for an unlimited family plan for 4 years, that's a few hundred dollars to backup 10 systems. That's about what you'd spend on the hard drives alone for a local backup solution, but with lots of other benefits.