Entourage backup is easy. You can backup everything, including settings by simply backing up one folder. If you have to reformat, you simply replace this folder and all of you settings and emails are restored.
In the Home folder, open up Documents.
Inside Documents, open Microsoft User Data.
The folder Office X Identities is the one you want to backup.
If you want to save individual folders, such as inbox, while Entourage is open, simply drag the folder, such as Inbox to the desktop or a folder or a CD-R from the side pane where you choose which folder is showing.
The backup of the Office X Identities folder is easiest since it preserves settings. Either way, have fun.