There has been discussion of using the Apple Flasher and Applescript to do the update, but the one guy who tried it reported some issues which made it appear that it had failed. Power the Mac down, swap cards, reboot. The flasher runs nonetheless. Set it as the primary monitor. Shut down, put in the MX card, reboot.
Video comes up on the PCI monitor.
AGP card in the AGP slot. Both scenarios also are a one-way ticket, in that the PC ROM image is not captured. If that works, you're done. Hz and 256 colors. I tried resetting the motherboard to no avail.