You will have more resolution on both D90 and D7000, however that doesn't tell all about the IQ. D600 will blow the socks of D90 in every aspect, and as well of the D7000 even with 16mp.
That is IF you use DX crop mode on the D600, if you stay in FX mode you might just get a larger image than the 24*16 DX gives you. Some of you DX lenses might cover a larger area than 24*16, I think it's 35mm that is a good example of a lens that covers a lot more. So what you could do is to disable auto DX crop mode and see what you get, you can always crop i PP, in LR4 you can even easily do it in a batch (sync) so that's pretty easy to optimize for each lens you are using.
But - theres a big BUT - you wont be really happy before you have FX lenses for that body