I also have a NAS that I bought specifically to back up 3 computers and to store my photo library on ( mine is about 80GB). You can also back up the NAS to a separate drive or you could get one running Raid.
At the moment I have my Mac backed up to it using Time Machine, I have the photo library on the NAS and also a USB connected hard drive. I suppose to be safe, I should store the USB drive somewhere else.
Allan