Here is a good comparison. It doesn't have the Nikon D800 though.
Scroll down about halfway down for the comparison images.
http://www.imaging-resource.com/PROD...-5d-mkiiiA.HTM
More importantly it shows 5D mark ii versus 5D mark iii up to ISO3200.
It makes it seem though that there isn't a massive difference whereas I saw another high ISO comparison where the 5D mark iii had clean images at ISO12600 which would be extremely useful for astro landscapes/time lapse.
Whatever the improvement in noise, its still going to be the better camera against the earlier model. Just how much better is for you to decide based on your needs for a camera.
Greg.