According to DXO the noise performance of the D4 is about .6 of a stop better than the D3X up to ISO 3200 where the D3X curve stops. So the noise at 1600 on the D3X is about the same as the D4 at 2600. I think you're going to see noise with both cameras at a 16x20 size print. But don't forget a print that large is always viewed at a distance much greater than what you see on your computer screen. The secret is good focus, no under exposure, no camera shake and minimum cropping. A little noise reduction won't hurt either. I shoot at ISO 1600 a lot with my D3X and am quite happy with it. It's much better than most people think.