As a hard disk is the fastest and largest backup medium you can get, you may want to backup all your important files to an external hard drive. You can backup literally everything: your Outlook and Outlook Express files, your bookmarks, your favourite save games, and of course, your important work documents, photos, images and maybe even videos.
As an external hard drisk drive is detatched from your system and, combined with a strong encryption algorithm, like blowfish or 3DES, you can get maximum security. Just make sure you actually remember the encryption password. You can also store your external hard disk offsite (at a friend's house, a paid locker or specialized fire-proof storage place) in case your information is very important (well, generally, anyone can do that and it can't hurt for sure).
As external hard disk drives have the cheapest storage volume (about 2Tb for $150?) and you can reuse them for several years, you may as well get the cheapest backup medium at your disposal. When using a backup schedule, you can simply backup the backup program run when you are absent (or maybe even at night) to backup everything without supervision. If you need two or more backup disks, you may as well get another external hard disk drive.
Also, now there are quite large mobile hard drives, which can admit all the necessary data: work documents, e-mail files, etc. You can also carry your data anywhere you want and feel safe, because some external hard disks are now hard to break by dropping.
So, go with external hard disk drive and backup as fast as you want, as much as you want, secure your important data, get the cheapest backups and even be able to carry your backups anywhere you want.