A 7800 prints at about the same resolution as the 7900, but with different color abilities. A 9800 or 9900 (or even 11800, I don't think there is an 11900) has more physical area to print upon.
I don't have a D800 so presumably I am not qualified to comment - except that I do have very large format scans, from 4x5, 5x7 and a few 8x10 negatives, which result in ridiculously huge 170, 200 and 350 megapixel scans. I also have some even bigger files, scanned from larger originals, which are as big as 770 megapixels (gasp, not a typo, that's seven hundred and seventy megapixels). These make the 36mp files from the D800 look positively tiny by comparison...
I think that it really boils down to how big you want to print the files. We scan at high resolution on good scanners because at the end of the day we're in the business of historical archiving and we are in effect converting from physical prints and negatives to digital archiving. And we have extremely limited manpower, so scanning once and never touching the original again is an attractive proposition. It's hard to conceive of why one would take a 400mp file and need to downsize it to print it at "only" 11x14 or so, in other circumstances.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!