Vista - backup suggestions?

I'm using Windows backup in Vista ultimate for scheduled backups to a
shared hard drive on a Linksys NAS. After an initial full backup, I'm
using the option for new and updated files to be added to the backup.
The same setup is in place on a total of 3 computers in my home.

The problem is the size of the backup is growing continuously. Can
someone recommend a strategy for complete automation in a manner that
overwrites older backups after a certain number of cycles?

I also have Windows Onecare but it will only backup unattended if an
administrator is logged in at the time. That's true despite storing
the administrator user ID/password in the backup setup. (DUMB DUMB
DUMB DUMB DUMB DUMB ...). Windows backup saves the administrator ID/PW
as recorded and runs without regard to the status of the user account
logged in. Even if WLOC backup used the stored ID, I don't think it
would be any different with regard to the backup strategy.

In addition to the scheduled backup, Vista makes shadow copies. I
believe that provide a modest amount of local protection but does
nothing for local hardware failure. The shared hard disk is currently
a single hard drive but I'm going to buy a match to it. The Linksys
NAS will copy disk-to-disk once daily, providing additonal security.

Thanks for any suggestions, including moving this thread to a more
appropriate forum.
--
Chris Cowles
Gainesville, FL