I actually have HP's Media Smart Server, which runs Windows Home Server.
It has 4 drive bays, one is full already with the OS and some storage space, the rest you can fill up as/when you need to. It does also have some e-Sata connections and USB connections, but I dont use these.
The server then does two things for backing up.
Firstly, there is a small client which you can install on the other PCs on your network. This client then backs up each drive on it's host to the server (Resulting in each pc being completely backed up every night). It will then flag any issues with backups and ask you to remedy the problem.
Secondly, it uses folder duplication.
It allows you to easily create shared folders on it's drives, and you can tick a box to duplicate the folder. If this is ticked it will keep a copy of the folder on two separate drives in it's bays.
The best part being that since I installed it, I've hardly had to do anything at all.