Back up your stuff!

Being the victim of a hard drive failure is no fun. I know first hand. That being said, I strongly urge anyone with data they can't afford to lose, to have a backup system.
I highly recommend investing in an external Firewire Hard Drive. Firewire lets you transfer data at a sustained data rate (where USB 2 does not). A USB drive is better than nothing and will still work. Combined with the hard drive, you should download a program like Carbon Copy Cloner. This can make a bootable backup of your internal hard drive and be set to backup your drive at regular intervals. This is a more manual way of backing up, when compared with Apple's new Time Machine, I personally like to know when and where my data is being backup up to. There are lots of ways to back up. Just make sure you have a way. Remember, If you don't have 2 copies of a file on different mediums, then you don't really have any.