1. Yes - I use up to 6400 without much thought.
2. I don't really crop that much, but it seems fine to me when I do.
3. It's far superior to the 5D mkI, can't comment on the mkII.
4. I have a 5D and a D700, bought the D700 for the AF and high ISO.
The D700 doesn't disappoint, but at the same...