The new D800 measurbation samples have been out at the IR, now we can use their "comparometer" to measurbate and "compare" endlessly!
http://www.imaging-resource.com/IMCOMP/COMPS01.HTM
Choose 645D and the D800, the match begins, for a few examples (Click on the following to view samplein full size in new tab/window):-
645D, ISO 100, 75mm, f/8
D800, ISO 100, 70mm, f/8
645D, ISO 800, 75mm, f/8
D800, ISO 800, 70mm, f/8
Now, I can safely conclude that the 645D is endangered! At less than 1/3rd of the cost of a 645D, the D800 actually lives up quite well to the 645, despite that I think the 645D images are still slightly better in IQ at *low* ISOs. Needless to say more about the rich feature set, better portability and much higher system performance of the D800.