8 pins for data RAM
and others for addressing RAM and control.
That's sort of correct BUT you have to have the right pins if you want real XM access, and the Arduino team did not see fit to provide them so you will have to dick with the board and solder wires in place.
You can of course bit bang the interface using any old pins but that will be incredibly slow in this context.
As I said though external RAM has been implemented by someone but I can't remember by who or where the post is.
IMO the Due as it stands is not an appropriate platform for hi-res VGA, but if you can properly implement XM it might be OK.
Rob