> Will my D3s print a high quality 20 x 30 print? Yes, it will. How much larger will the D3x print?
I have done 24x36 with my Leica M8 (10MB, 1.3x factor) with excellent results, so I would think your D3S should be easily capable of 20x30 prints. This all depends on how you stabilize the camera for the exposure of course. I would not spend the money on either the D4 or a D800E as you already have an excellent body in the D3S.