For what it's worth, I use CrashPlan on one of the machines you will access the NAS from. It's the cheapest I've ever found for unlimited backup. Some people configure a Linux distro in a jail, but that also adds complication in, and of itself...
Using this for home use myself, what I've done is to have a snapshot of my datasets created depending on my preferences (yours will differ) and tolerance for loss. For example, your documents might take an hourly snapshot, but your movie collection might take a snapshot once a day or week...
Then on top of this, set up CrashPlan on the machine you will most frequently access the server from, and point it to backup your mount points (NAS data). Whenever you log on to that system, CrashPlan will copy the most recent changes to an off-site backup with no limits to storage and version history.
Not perfect, but works for me. I believe I've researched hard to correctly set up my NAS to be fault tolerant, and this is a good balance for me. Consider how much you're willing to lose if the machine just randomly explodes. If you can lose the last couple days of changes and revert back to your last online backup, then plan accordingly.
Now because I don't trust anything completely, I will likely buy an external USB hard drive and add a reminder to my calendar to copy the NAS data over once a month or six months, whatever I decide. This way if CrashPlan were to have some glitch that prevented recovery, you'd have a Plan 'C'...
Good luck.
Steven
