As mentioned earlier, I tried that. The controller is apparently not what XP dislikes about this machine.
MSDOS works with or without the RAID card, and works with the BIOS mounted RAID drive. In fact, MSDOS will boot from the RAID drive without the IDE drive installed. Of course, running MSDOS on a 700G drive is pointless.
Windows 2000 Server works with or without the RAID card, but for some reason the driver for it fails upon boot. I could probably figure that out, but I really don't care to run 2000 Server. I only installed it to make sure it worked, and to see if XP could upgrade from it, which it cannot.
I didn't supply the 2000 Server installation process with the drivers, I installed them after the fact. I think it's very likely that 2000 Server will install on the RAID drive, especially since that's how this ($10000) machine shipped.
For what I want to do with this machine, it must run XP. At this point it seems that it just won't. I've got a few more things to try, otherwise I'm either going to cannibalise this thing and "recycle" it, or just put it back on the shelf for another 15 years.