While the best procedure would be to backup your entire system to an
external device (or second hard drive, or even a second partition), backing
up just your personal files to external media (burn on a CD/DVD for
instance) may be sufficient. This is usually done as a precaution in case
the install trashes your system and you cannot recover from it to get your
files back (which is a rare case). Typically, anything you'd miss or isn't
available elsewhere is a good candidate for backup. I usually backup my
entire home system once a week to a second hard drive and that is good
enough for me. In truth, having a backup of anything you wouldn't want to go
missing is a good idea even during normal use, since hard drives do fail and
OSs do crash.
Others may have alternate suggestions for you, so be sure to wait for other
replies. In my experience, backing up personal files tends to be the most
efficient (although perhaps not the safest) course for general home users.
