If you are interested in seeing what the Nikon D810 is capable of in terms of image quality and ISO performance, take a look at the below high resolution image samples. These images demonstrate the sensor performance from ISO 64 all the way to ISO 3200. EXIF information is included with each shot.
Judging by the Sample Image #4 below, ISO 3200 looks pretty clean and impressively detailed. And ISO 64 images (which there are a bunch of, look quite detailed and rich in colors. Please note that these sample images are straight out of camera JPEGs, with no post-processing applied.