Stay the course or buy the right camera and equipment for your needs.
Edited to add:
The original post raises the classic NEED versus WANT dilemma, compounded by the GOALS question. Most people are unable to effectively state which category they fall into. By observation, most people fall into WANT, not NEED. And they can't tie that WANT into how it will really achieve their GOAL. I don't know which category you fall into, and quite frankly that's not really material to this discussion. My point is simple - most people, after a long and sometimes painful introspective look, realize they're NOT limited by the camera, full stop. I happily fall into this category.
It's still a free country, thankfully, so people can still go out and buy whatever even after coming to the conclusion that it will not necessarily solve their current gaps. As for myself, more post-processing time/instruction, and trips to take more photos, are what will really make my photography better. Either way, resources will be expended, but I have the small comfort of knowing that my path is more cost-effective to getting to my goals.
If you really think the D600 solves your problem, then bully for you. That doesn't make the "equipment vs. other investments dilemma" for the average photographer any less valid.