>The D200 can't get to 13x19 inches without upsizing by quite >a large amount - for instance, at 300 ppi the native print >size is 8.64 x 12.91 inches. The D800 on the other hand will >go to 16.37 x 24.53 inches without upsizing - very nearly >twice as big in both directions.
When I start talking about things like storage capacity, processing power, glass and lighting, I'm moving away from the thought experiment and taking the example into the real world. Really by limiting this to 13" wide I've been extremely conservative in what I've stated above.
The truth of the matter is, your eye can't resolve 300 dpi at a typical print viewing distance, many subjects don't contain this level of detail anyway, and moderate amounts of upsampling and sharpening will provide detail plenty adequate for printing this size.
My experience tells me that with quality pixels from capture to print, it's almost impossible to differentiate between 6MP and 12MP in a 13x19" print, despite the apparent lack of resolution of 6MP. I'd say it's safe to extrapolate from this, issues of noise and dynamic range aside, that 36MP won't yield a significant improvement either. This is why I mention D700, which will do a better job leveling the noise/DR playing field than D200.
I stand by my summary above. Your D800 either comes with a really big printer, or you're not going to realize any benefits over cheaper cameras already available today.