The concept of game resolutions does not really apply to TVs using S-Video, because the VGA card just converts the screen resolution into the TV resolution, which is always the same.
So this means that two sets of conversions are going on. First, MAME is stretching the game to fit the chosen res, eg 640 X 480, and then the VGA card is converting it to the TV resolution. Which is why S-Video TVs are so horrible for MAME. SCART RGB is a different matter of course but that's Europe only...