Nothing fancy, just internal. And I back up between the drives.
OS and programs on first drive. Homedir on the second. Media on third. Business on fourth.
OS and Media aren't being backed up, confidential details, preference, and passwords, etc are stored using info.xhead and 1passwd (and synced over network). Homedir drive and business drive are being backed up. The business one actually also offsite).
I am not using raid or anything like that, because I switch hard drives too much. If I get a new drive I want to copy the data over, swap the spots and move the slowest/oldest/smallest one to external firewire800 or usb2
I don't need a NAS, I have FTPd on all systems which takes care of everything (and anything big if i need it) And network is 1gbit. Most computers are on 24/7 anyway, especially the mac pro and the ubuntu pentium server