And if the sensor density of the D800 is the same as DX (which it effectively is), and if the inherent noise performance of the D800 is essentially the same as any other current camera (which it is, per the testers) then you have the same film stock in DX or FX.
And at that point you are back to deciding if you should shoot with standard 35mm film or a Nikon Pronea. It's not the rocket science everyone seems to try to make it in order to (hopelessly) try to solve their problems with even more pixels from a smaller sensor.
There are many very good reasons to shoot DX, but a lot of the issues brought up here are simply not those good reasons.
Ironically, many of the reasons people have for upgrading the D300 are the very same reasons why FX makes more sense. But I think I'm done arguing that battle because it is obvious many do not want to recognize the facts and the physics.