I've never actually had a bad crash before. That being said I'm terrified of it actually happening. I currently have a mac with my most important files on it that backs up hourly to a 1 TB external drive. Because I do agree with Hack's suggestion that a drive shouldn't run constantly, I then have a 500 gig drive that I plug in once a month to backup anything new and important I may have acquired. This drive stays in a closet when not in use.
This is my dream set-up:
http://www.drobo.com/
Essentially, it's a casing that holds up to 4 hard drives but shows up as one giant drive on your desktop. anytime one drive begins to fail it auto-dumps all your data to another drive. Once the drive has died you swap it out for a new one and all your data is safe. $500 though. I'd love to have one.