Mailbox management policies are your best firend. If you are archiving then why do you need to allow users to keep all their mail in their mailbox? They have access to their archived mail don’t they? I would suggest you implement and enforce a retention policy. Ours is 15 months. Anything older than that is automatically deleted. Since users can access their archived email that is where they go to find older mail, saving us space on our Exchange servers. We have just a 30 day retention on the deleted items folder. If you move it to your deleted items, then why would you need it after 30 days? Users are lazy and don’t take a moment to empty that folder otherwise. I found one user with nearly ALL her mail sitting in deleted items, and sorted into folders even! No more!
The best thing you can do to backup your entire database is to use incremental backup using a third party back up software. You really need another hard disk drive which has a large storage space for your future files to backup.