A common question I seem to be getting since Nikon Rumors pre-announced a 36mp D800 is this: should I get a D700 or wait for the D800? A variant of that question is: I have a D700, do I really need a 36mp D800?
Most of the nervousness comes from all those pixels. But if you're worried, here's a little experiment to try at home. Borrow a D700 and D7000. Set the D700 to DX mode. Photograph the same exact composition with the same lens on both cameras. Now downsize the D7000 image to 5.4mp. Compare away. Extra credit: use the same zoom lens with 1.5x focal length differences to photograph the same scene with the D700 set to FX crop, but downsize the D7000 to 12mp.
We can't isolate all variables when we attempt to make image quality comparisons between two competing models/products. But we can often isolate enough to make an intelligent decision. The D7000 has about the same photosite size as we'd expect from 36mp FX. Any new sensor might be even better at some underlying dynamic than the 18-month old D7000 sensor, so you're probably looking at worst case in the above comparisons.
Of course, the next problem is that some people won't notice any differences, while some will. The question is this: if you can't see the tree fall in the forest, did it fall? Or worse still, quantum mechanics might be involved if you don't actually perform the comparison: if you don't look in the forest, the tree might or might not have fallen, and which of those is true won't be determined until you look.