I would be surprised if the D800 sensor somehow magically performs much better on high ISOs than the D7000 sensor, seen per pixel (they have the same pixel pitch). My guess is that it will perform more or less exactly the same per pixel, and gain advantage from the large area and resolution. Sensors are evolving slowly now, don't expect a revolution.
It will compared to the alternatives be fine up to ISO3200 (don't forget noise averaging due to the high res!), then there will be significant drop in performance, poor color reproduction at very high ISOs will in real prints be more of a problem than noise. If you're a ISO6400 - ISO25600 user then this is not the camera for you, then you need D4 if Nikon user and 1DX if Canon.
The D800 sensor is a high res sensor, so I think it would be much more interesting to look at what it can do at base ISO, and it will be much interesting to see how full-frame lenses perform resolution-wise in the corners.