ETA (can't figure out how to edit previous comment): the video card appears to be a SuperMac Spectrum, although the Monitors control panel (when I finally got it working) gives no further modifier (just "Spectrum") - perhaps the earliest model, Spectrum/8? 

Having no luck finding the appropriate drivers. The SuperVideo 2.7.4 control panel appears to have crashed my HD with 7.5 installed (then again, for all I know, the drive had other problems).

I wonder if it might be easier just to shop for another video card, preferably packaged with drivers...