I have never gotten the oppotunity to try a D800. Pretty sure that batteries would match the ones in my D7000. Not too fond of the 36mp size, but the 15mp for DX would be fine. For sports, I would like a little better frame rate.
Had no clue about the smaller view in the viewfinder. I wonder if the DX view in the D800 viewfinder is smaller than the normal view in the D7000.
This may truely be the time when a rental would be appropriate.