[K12OSN] offsite backup?

[Paul Lemke]

Hi all, 
So i'm working with a non-profit to get a network going. We have pretty much
everything taken care of except 1 thing... offsite backup. Currently we have
1 server with a 160gb raid hdd. Currently they have a 5gb tape drive...
which obviously isn't going to work to backup the 50gb I'm estimating for
their home directories. Just glancing at tape drives... they seem really
really expensive. 

So I've only come up with a few solutions. None of which i like. 
1. purchase a tape drive - seems too expensive 
2. some kind of online backup - seems too expensive, and the place is only
on a slow cable connection so upload is not very fast. 
3. burn a few DVD's every week. - I like the "snapshot" but someone would
have to sit there and plop in a new blank dvd when it takes more than just 1
blank dvd, not to mention the reoccurring costs of the dvd's.
4. USB Hard drive - fast, easy, cheap (saw a 320gb 1 for $150 today), but
the reliability is an issue. 

Out of the 4, I like the USB hard drive the best. I'm just worried about
reliability. I can imagine the secretary taking the hard drive home with her
and dropping it on the floor and then it's busted. Or it falls off the desk
or something and they are screwed. 

So the questions... 
1. Is a USB hdd backup a reliable solution? 
2. What other alternatives are there that i'm missing?

And what software would you recommend? 

Thanks!
Paul