Your first problem is that the Due doesn't really support the external memory function because they didn't break out all the pins. You can kludge something up but it's just that, a kludge, and I can't remember what (if any) limitations there are with that approach.
One of the guys has a thread about it on the forum here.
As for the timing I'm not sure, but as you can see it's working with internal RAM, external will be slower but I would think still fast enough.
One question is what will you do with all that RAM, it's not required for a text display or even a lo-res graphic display.
Rob