one that works?
dual drive I would say a raid 1 setup but really a raid 1 isnt significantly safer than a single external drive. Why? because if the controller/surge/fire/explosion/ tornado destroys the HDD it will kill them both.
Your better off with two external drives and making 2 backups and then keeping at least 1 of them off site.
Best software-wise: Windows Home server 2011 ($50 on newegg). Build yourself a cheap server and WHS will handle backing up all the pc's. You'll just need to backup the server then. (yeah, no easy way out.LOL)
A drobo box also gets high marks but its rather expensive in my opinion.