Here is a test image at ISO 6400. I also have attached a 100% crop. This is straight from the camera - no noise reduction or sharpening is applied at all.
As you can see, with just normal sizing ISO 6400 is completely usable. The extra pixels of the D800 are worth about 1 1/2 stops through resizing to match a D700 sized image. Even without resizing benefits, ISO 6400 is completely usable.
The downside of high ISO's is you lose the big advantages of dynamic range and color rendition.
It's not just the D800 that has this kind of advantage. The D600 is very good as well.