(OP) If you are not making large prints, like posters, you aren't gaining anything with the D800..
I have seen/made gallery prints taken with everything from 5D, D700 to 5DmkII and D800, and unless you are talking about the finest detail that can't quite be reproduced by upscaling a 21/24mp image for poster prints you aren't going to be able to tell any significant difference in the output other than the difference created through post-processing.
If you just want to have that marginal detail whether you print big or not, like just in case you have/want to print big, then pick up something with higher mp...
I mean at the upper end of landscape photography, MF bodies, its not like everyone who shoots MF no longer owns or uses a DSLR. No capable mechanic only owns/uses one wrench...
Also remember that it is the accurate capture and reproduction of light and color that resolves detail, not mp.. but mp do give you a digitally larger image file.
(Note to Nikon trolls - your teenage Penthouse forum fantasy tales are boring.)
it's the post work, in some cases, that benefits from the reduced pattern noise afflicting some Canon bodies
..and it's more like missionaries