You could be talking about an STE one that is around, this interlaces the screen giving much higher resolution (high is then 640 x 800). This program may require a higher TOS version than yours but you will also need it to be an STE.
I have also found that Med and High are the only reslutions usable. Increasing the resolutions will not help much for games and such and I found very few programs written well enough to take advantage of it, many more will work but it makes no difference.
RE: Rez booster for st 