Yeah, well, it was easy enough to get RAID working. Sort of. Here's the compressed notes.

http://www.linuxhomenetworking.com/linux-adv/raid.htm gave me the basics: cfdisk /dev/hdb and /dev/hde to one big partition of type 'FD', copy a sample config from /usr/share/doc/raidtools* into /etc/raidtab and edit it, mkraid /dev/md0, look at /proc/mdstat to make sure RAID was active, mkfs /dev/md0, go eat dinner, remember raidstart and raidstop commands for future reference, put /dev/md0 in /etc/fstab, and mount -a. Now I had RAID. But it was resyncing very slowly!

hdparm -i showed me that one of the disks was in mdma2 mode, which was painfully slow (as shown by hdparm -t after a raidstop.) Also a little inspection showed that I did actually have a third IDE channel (praise be unto server-class motherboards). hdparm -X udma2 /dev/hdc (it was hdc after I moved it to the new channel) helped a lot.

Very painful lesson: make sure you set AUTOSTART="true" in /etc/default/raid2 on debian systems, if you want your RAID to be mounted at boot time. (Otherwise maybe use noauto in /etc/fstab.)