Wow! This is a very good and comprehensive analysis. Thank you very much.
One new point that got my attention now is DOF using a 5.6 vs a 2.8 lens. My experience so far has been with the slow lens and I feel that I'm getting a good balance of focus on my kid vs the others. If I go down the road with a new 70-200 2.8 (either for the D90 or a new D7000), I will be able to shoot at ISO 800 or so and also increase shutter speed a litte, but I am now clueless about the impact on DOF If I follow the road of the D600 using my current 70-300 VC at 5.6, I will probably get a similar or slightly worse improvement on low light capability, but will DOF at 300mm/5.6 be much different on the FX compared to what I am now getting from that same lens at 200mm/5.3 on the DX?