I've written it before, and I'll repeat it again before I present my detailed recommendations: it usually doesn't pay to update from one Nikon DSLR to another every generation. For the amateur cameras skip a generation in order to save money, as single generation changes tend to be modest.
You'll note a slight bias against moving from DX to FX in my guide. Realistically, not everyone needs FX, and the FX cameras are more expensive. Indeed, when I quiz people asking the "should I get a D4/D600/D800" question about why they want FX, most of them don't know. In their minds those cameras are in some way vaguely better, but they can't define how it will improve their picture taking. If you can't define it, you don't need it. (Yet. You may some day.)
It's easy to get so caught up in generalized absolutes (e.g. "bigger is better") that we start believing them without challenging them. Let me put it this way: if you haven't printed larger than 13x19" or shot at ISO 6400 lately, there's absolutely nothing wrong with DX. Even if you have done those things, FX isn't an automatic choice. I know many pros that shoot DX. I've done it myself whenever I'm not doing landscape photography (36mp in the D800 is hard for a landscape photographer to pass over) or indoor sports (the D4's high ISO capability is unmatched for indoor sports work).
Before reading the appropriate section below, may I suggest your first read this article?
With that out of the way, here's my detailed recommendations for every Nikon DSLR user (the rest of this page is an accordion: click/tap on the colored topic headers to open and close them: