Weekly data backups are a good idea. Backing up your stuff when before you go on vacation is probably a REALLY good idea!
At Rainy Day Magazine, we have more than a terabyte of hard drive space inside the G4. What do we use to back up the critical data? A Maxtor external Firewire drive... of course!
The nice thing about using an external drive for back up is you can easily unplug it from the system and LOCK IT UP! Something that is hard to do if you back up to an internal drive.