WHDLoad, the games, the system need some more or less memory to work. How much do you need depends on this. Personally, I would suggest an AGA Amiga (i.e. 2 MB chip) and >= 16 MB fastram to be able to play all (I guess so) WHDLoad games. So you don't need to care about. 8 or 4 MB fastram might be enough but that depends on games you want to play (only OCS or also AGA).
Workbench/OS will not bring higher graphics resolutions. It's a hardware thing.
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