In my experience with the D800, I'm happy at anywhere from 100-800 and if I need to even 6400 and 12,800 if it's between getting the photo or taking home just memories.
Even at ISO 6400, I've been quite impressed with the dynamic range and with the NR in ACR, you can make a pretty clean file from these nose-bleed ISOs.
As always, you need to test the camera at the different ISOs and see what works best for your own tastes. But, don't be afraid to push the limits upward for both effect (more grain-like noise), or to get photos under hail-Mary conditions.