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With D3 - A3 can be printed at 300 DPi or so.. from what I saw, A2 isnt problem, but it depends what you shoot. If you keep it at base ISO, it can be printed pretty crazy big, cause its per-pixel clean.
Tho obviously, upon close inspection if you would compare A2 prints from D3 and D800 you would see difference.
Its bit complicated matter, cause it doesnt depend only on camera but on print resolution/quality too.
Advantage of D800 is, that you can squeeze that resolution to smaller print too and if you do resizing right and print in high resolution (350 DPi) it will be visible. If you look close enough.
I would say that D800 for landscape is pretty good choice, if you intent to print big. If you want to shoot "almost everything", then D4 or D3s or even old D3 is enough.
12 mpix isnt much of limit, when its per-pixel sharp and clean picture. I prefer low res with high per pixel sharpness/lowest possible noise over high resolution and noise.