Quote Originally Posted by davewmerrill
....... - 1 power supply (if it goes then you're done)...
There is a difference between the supply failing and loss of data. If it's only a backup system then the supply failing usually will not harm the drives, and you can replace the supply and hopefully have your backup system back. But for really mission critical systems I have built redundant drives each in their own case with their own power supply and multiple computers (SCSI rather than IDE for the external case support).

Quote Originally Posted by davewmerrill
.......The reason I was thinking of doing this w/a live CD was that you could get an extra drive in there (4 data drives rather than 1 os drive and 3 data drives). ...
Well, if this is your basic old system with 2 IDE ports then you are not going to be able to have 4 IDE drives and an IDE CD drive. I don't see any need for keeping the OS and data on different disks, as long as you can boot into the raid system. If not, then an OS drive or at least an OS partition would be needed. If you have enough IDE connections for 4 hard drives and a CD drive (by my count that's 3), you could slave another small hard drive onto the IDE connection supporting the CD and use that for the OS and still have your data RAID.