What is it you will be looking for in your next camera? Will it be a replacement for either of your current cameras, or an addition? How often do you need/want to shoot at more than 6fps?
I wouldn't worry about noise levels from either D600 or D800 - if you're happy with the D7000 and D700, you should be happy with either of them on that front. And personally, I wouldn't let the hysteria about AF or dust problems affect my decision.
But from the information you have given us, I would suggest that a used D3s might be a better fit than either D600 or D800. You'd get the higher frame rate and even better noise performance than your D700, without the apparent penalty of larger file sizes.