The first time you run Cro-Mag Rally you will get the Video Mode dialog:

The game defaults to the settings you see above. You must be very careful about changing these settings since any change from these defaults will require more VRAM. Macs with only 6MB of VRAM must use the above minimum settings or there will not be enough VRAM for all of the game's textures. Performance will also be affected. The higher resolution you set, the slower the game will run.