I have a similar problem and solved it this way:
I got an old Compaq server from e-bay. populated it with a bunch of high
capacity SATA drives and a SATA controller. Some limited sheet metal
mods and a bit of velcro made it all fit.
Next loaded Linux on a small drive, 40GB. Made a soft RAID 5 array out
of the other drives, 2TB.
I setup RSYNC to run on different places and put them in the appropriate
I setup a RSYNCD in each of the systems I wanted to backup. There is a
great Windows version.
The system has been on line for a year. One SATA drive died and I
hot-swapped it and continued on.
The key is cheap drives, rsync and Linux. The whole deal set me back
less than a tape library.
You might also like backuppc (http://backuppc.sourceforge.net/). It can
use tar, smb, or rsync to for the backup tranfer, then compresses and
links all duplicate files to save space and allow keeping a longer
history online. It also has a nice web interface to browse and restore