Thanks, Inaris. In fact, the boot order was correct, but the board was trying to boot from something that wasn't there, as you guessed. I disabled the RAID controller in the bios, and allowed it to talk to both P-ATA and SATA drives, and I was able to boot.
I think the 'bios missing' bit is a problem with RAID that now has a fix on the Asus website. I flashed the bios sequentially with all the available fixes, including this one, to avoid problems in the future.
I now have a working computer!
Thanks for all the help, guys.