I don't print exceptionally large on a regular basis, however, I have pushed other cameras with lower resolution a little bit. Professional printing is considered to be 300 ppi (not dpi, that is dots per inch and most modern printers print several dots for each pixel) and it is usually considered that 240 ppi is fine for almost all images. If you have a high quality image, it would probably look ok at 180-200 ppi, but that is where it start to depend on the image itself, not just on image quality, but also on the type of image. The largest I can print is A3+ and the largest I've printed at home is A3 (42x29 cm, roughly 16"x12"). I have also had images from the 5D MkII and 40D printed to 28"x18", but that was on canvass, where you can print at lower resolution. Images I have printed from the 40D at 200-240 ppi haven't shown any noticeable deterioration in quality. I'm sure if you started looking at it in minute detail, you could tell the difference, but for normal viewing, it is fine. I can't answer the rest of your questions, as I don't have the D800, but if you extrapolate, you should get some idea. By the way, printing on the 5D MkII/III would give you about 135ppi at 42"x28", which would be ok for canvass, but probably too low for standard printing and personally I would aim for 200 ppi as a minimum, but would drop to 180ppi for some prints. If you are printing that large on a regular basis, then the D800 would theoretically give you better results.