How mission-critical is it? What are you backing up?
Backing the server up to itself (a second HD) is always great (and inexpensive). To take it further, have a backup server also backing up that server (and all your others) with a RAID5 array: a 4x120 array that only uses 3 drives with the 4th being on standby in case a production drive fails.
You could get a second backup server too, if you wanted to. And put it offsite.
Just ideas...