The Nikon D700 and D800 are indeed different cameras serving a different purpose at times when pushed to their limits. The D700, which I own, is an outstanding low light performer and will go down as a classic along with the D3/D3s model cameras.
The D800 is not a D700 on steroids as some have said. It is a different animal that provides results that can differ dramatically based on the challenge presented to it. Even within the D800 family there are variations with the D800E being offered as an alternative solution to photographic challenges. It is a powerful image capturing tool which compliments the existing models and offers an additional choice for those who may need/want it.