You may want to consider keeping the mirror drive out of the machine, to eliminate the risk from co-location (e.g., power surge, the computer falling over, or other things that could knock out both drives simultaneously). Right now I use a portable 1 TB Firewire drive that's stored elsewhere in the house. This makes it extra convenient if I have to evacuate the house for some reason. I also keep a secondary backup at a relative's house and update it every few months.
I've looked at RAID options, but there's that co-location issue.. one thing I might try is putting the redundant drive in another computer on my network and see if there's some software that will mirror the drive over the LAN.
Tim