Body or lens?
That's a question. It is true that d800 can produce better downsampled 12mpx images than d700. However, I don't believe that D800 with 24-120/4 cropped could produce better images than d700 with decent true 200mm lens like 180/2.8D or 70-200.
I would get better lens if you need more reach. Even cheap AF-S 70-300 3.5-4.5 VR get you far more reach than D800 with average 70-120/4.