tks about your answer Graynomad,
Of course external RAM will be slower but may be enough to be just a bitmap.
If it's the case we could display a resolution better with more colors.
If you have a RAM you make a kind of double buffering.
About pins I read that Arduino Due have 54 digital I/O pin.
I don't know very well this product but I'm imaging :
10 pins for vga
8 pins for data RAM
and others for addressing RAM and control.