Weird update. Tried using the PCI and AGP cards in tandem a few more times, swapping between which one was connected to the monitor every time I did a cold boot.
And kerblammo, the Matrox worked. Once, briefly, then was back to not working after I reconnected the SB32 and changed my master/slave config with the disc drives. I went back to power cycling and swapping the video cable, and sure enough I can get the Matrox to work if I just TRY HARD ENOUGH! It seems to continue working through warm reboots, and even power off, as long as the power supply remains plugged in and switched on.
So instead of a failed card, I've got a failing card? Or could it be a power issue?