I shoot a D300 and a D700 and I range from Sports, Wildlife, Landscape, Concerts, Portrait. I use my D700 most of the time except for wildlife being the crop factor gets me closer. I do switch to the D700 when it gets low light to allow for higher ISO but using the D700 all the time and cropping is not as good as using the D300. My lenses are 80-200 2.8, 200-400 4 for most of my work and of course some fast primes. My question is being technology is of course improving sensors and MP is growing although, I dont think jamming more into the same size sensor improves once you get to a certain size. But would switching to one camera like the D4 allow me to shoot full frame but with wildlife I would crop and now have the same quality as if I was shooting with a DX camera. I like the D4 for its speed being when I do shoot sports I have the battery grip for the D700 which gives me 8 fps and it doesnt look like I can get that speed with a D600 or D800 and the battery grip. Which is something I need. so that is why the D4 is on my short list of "It's getting time to maybe upgrade my body and which one should I get?" question.