I will be utterly astonished if the D800 has a 36 or even 24 MP image sensor.
As more pixels get crammed into the same amount of real estate they have to be smaller. Smaller pixels gather less signal. Less signal means worse signal-to-noise ratios (SNR) or more image noise.
Look at Nikons 2 versions of the D3, the D3s and the D3x.
The D3s has 12.1 MP, the D3x has 24.5 MP.
The 12.1 MP D3s has the best high ISO performance of any 35 mm format DSLR. The D3s base ISO range is ISO 200 to ISO 12,800 with HI 3 at the equivelent of ISO 102,400, and is intended to be used by sports shooters.
The 24.5 MP D3x, Intended for use as a studio camera, has a base ISO range of just ISO 100 to ISO 1600 and few will ever use HI 3 at ISO 6400.
So what do you think the ISO range might be for a camera that has 36 MP? There have been no recent revolutions in electronics that significantly improve SNR.