I have never used a D700, but correct me if I am wrong here, but isn't the high ISO performance on the D700 identical to the D3? Don't they use the same sensor?
I am "expecting" or "assuming" that the ISO performance of the D800 will have at least the ISO performance of the D700 and thus what I am used to from the D3. I regularly shoot at ISO 2000-3200 for reception shots and have no issue with post processing. These shots are rarely blown up past 8x10" prints if if they need to be, they have enough dynamic range to hold up after a tad of post processing. And I can spend time doing this when I need to for large prints. Dimly lit churches often have some many restrictions on shooting during the ceremonies, that it is mostly a non-issue about noise. And I shoot at 3200 at a slower shutter speed when I am allowed, often with a tripod, and the only issue I have is stop motion when the B&G walk /run out. But by this time, most churches allow you do use flash.
Bottom line is if the D800 is as good as the D700 which is as good as the D3 in low light, the D800 would suit me just fine. I'd rather have the better focusing that the D800 should offer than my D3 focusing does.