#5. "RE: need buy a camera. D7100 or a D600 ???" In response to In response to 4
I did what you are doing, going from a D80 to a new body i chose the 7100 because i have a decent DX zoom that i could not afford to replace straight away. if you have FX glass or can afford new fx glass, go for the 600. if money is a concern you cannot go wrong with a 7100.
in my case, i had to go from the older body, it just was limitng. this way, I have a body thay will last a few years and I can take time buying good FX glass