When you install Calamity's driver and tell it you are using a 15khz monitor (I think you select 'CGA' as the monitor type), it will replace the 640x480 VGA with an interlaced version. So you just set the desktop at 640x480 and it will display fine (you don't need to go lower than that). Groovymame should automatically select the correct resolution for each game based on your monitor type setting.