If you're looking to really be safe....
Buy smaller and buy 2. I have a very large LaCie RAID, that I work on every day. I keep all my current (and older) work on it, on an ongoing basis. If any of the drives fail at any point I can pull it out, replace it and it rebuilds itself.
Beyond that, and what I think you're talking about...
Every month I take an external 500GB drive and backup everthing I've added since my last backup. Then I take a second 500GB drive and back it up again. I keep one of those at the office (not where my main work station is) and I keep the other at my Dad's house.
I use LaCie or Maxtor drives for these backups. The Maxtors or cheaper, about $100 for 500GB. My thought is, if one of those go bad, I've got 2 other copies elsewhere.
This may sound like overkill to some, but my thought is.... if my house burns down, I'm covered.... if the office burns down, I'm covered.... etc
My parents live about 5 house from me, but even if our side of town is taken out by something weird.... I'm covered.
Ever loose a drive, with your only copies of a photoshoot on it? If so, you'll understand... if not, you will someday and be pissed you didn't do something like this.