I think the D800 High ISO samples are better than the D700. Part of that is Sensor technology advances and part due to Expeed 3 processor, better AF, better white balance, better 3D tracking, and face recognition.
If I get the D800 and it does not meet my ISO needs, I will have $3000 to go toward a used D3s. It would be very nice to use as the Primary Sports Event Camera.
However, the D4 should go to more than twice the high ISO of the D800 and if I can come up with the money I will get one instead.