Then every month I'd like to take one of the full backups and put it away for long term storage.
My backup media is a 120GB IDE disks in a removable drive trays. Take them home for off site storage.
Are you reusing these drives or buying new ones every month?
One thing I'm trying now is: rsync the three servers to the local 120 GB disk. From local disk create tar files. Bad part about it is I have to store an extra 40GB on the disk of local copies of the servers. I can't just tar over NFS, because the files on the server may be changing as the tar is created. Tar bitches when that happens.
I wonder if LVM would help you. You can create a snapshot in time of your filesystem and back that up without worrying about anything changing. There are of course many more possibilities, see http://www.sistina.com/products_LVM_publications.htm and check out the SuSe whitepaper. BR