(Old Sierra Games System Requirements)
Go for a 486, 66 or 100. You don’t want slower than 66 for the newer games, but not faster than 100 for the older ones. Frans has a Space Quest gaming computer guide on spacequest.net. There’s another thread here about this sort of thing too. I suppose someone could document all the games and their technical specs, but I think most would agree that a faster 486 is what you want.