What I have is everything I care about comes from one of the drives in the DNS-323 (as in my system drives hold programs and such not my data). I have a script running on the DNS-323 that does link/rsync kind of back up from that drive to the second. That results in using the backup storage taking a little less then 4 times the space for the first drive for backups that go back to 2008-10-11. The same process also sends the changed files to Amazon's S3 storage a copy for online storage.
The beauty of the link/rsync backup is that I can jump into any of the the directories for a given day and see how my data looked on that day. No special programs, no waiting for data to be extracted or such. And it only takes the space of what has changed.