Judging from the date on the card, my guess would be 640x480 MaxRes too, but I haven't researched it. Might be a bad crystal since you're getting an almost legible display, but you'll need to wait for TomLee to show up or one of the MLA's other boffins.*

* IIRC, that was Brit slang for those magicians/sorcerers adept in the black arts of Electronics/RADAR/Electronic "Computing"/Code-Breaking and such at places like Bletchley Park, one of the pre-eminent Xerox PARCs of WWII.