If I am not mistaken, if you do NOT have one of the original MDD video cards installed, OS 9 will go wonky. Mine did with my 256MB nVidia FX5200. Mine would only display at 640x480? (I can't remember) and 256 colors. I put back in the original Radeon 9000 Pro, and it was fine.

EDIT - Wait... didn't the MDD come with the GeForce 4MX as an option?