I just read this older hint about RAID and partitions on OS X Server, and it seems that it does not work in 10.4. So I tried doing it differently.
I opened Disk Utility from the utilities folder and did the following. On the two disks I would be using, I created equal partions, i.e. all the same size. I named these partitions 1a, 2a, 3a on the one drive, and 1b, 2b, 3b on the second. Then I went to the RAID tab and did this:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060525033753937