Worth noting - when the database size exceeds 2Gb, entourage ceases to be your friend. Backups will drop back to previous versions of your mailboxes, but you'll probably have lost mail in this process.
Rule of thumb if you're running that close: COMPRESS THE DATABASE. Entourage caches *everything*, and until you compress, it's in there somewhere. If you get a lot of attachments, for example... yeah.
A note about backing up some mailboxes on Entourage: You can drag a folder onto the desktop and it will save it in a standard UNIX mbox format. Very, very handy and not well known.