Sorry,but why areyou useing 640x512i
Get yourselfa VGA-adapter (instructions can even be found on Aminet),
move DbPAL/DbNTSC,VGAOnly, MUTISCAN,EURO72 from storage/monitors
to devs:monitors, and choose a better res.
If that still ain't good enough (or you want really-old games/demos on the VGA),
get a Flickerfixer.....
Far from being as good as a real GFX-card, but also far cheaper and compatible.
