No old games use so high resoulution as 1680x1050, so I can see no reason why to pimb these games so high, so I think you should just use the half resolution for these games. It just "cost" cpu and/or gpu time for absolute no reason.
MultiFE can upscale to any resolutions with 1x1 pixel text and the baground is designed you really cant see it is upscaled (Apart from the coverarts of course), but it faw while ago I updated it, but I still work on it to get command line emulators to work (hence why I diddent update the MultiFE thread) aprt a very bug fixed releasese of the jukebox part.
Mamewah and Mala as minimum should not have any problems if you have 1680x1050 artwork, but these skins is not designed on these resolutions.