This wouldn't be a problem as long as you put a clear notation next to the "Gigantic" setup option and in the manual which says: "Caution, this option is not suitable for processors slower than 1.2 GHz. You will encounter too much processor slow down between turns."
Or: Could the program be made to detect the processor speed, and hence not even display the "Gigantic" option if under a certain speed?
But anyway, but as Soulfire says, "We'll just have to see" if the largest galaxy size currently in the game will suffice for our crave of a large galaxy.
One other note: There could be a "cap" on the number of races allowed in the game, to help minimize the processing power needed for the AI's.
And you could research a new tech in the game that would boost your processor. Would that work?