I just 'bought down' upgrading from D90 to D600. Well, hardly. D600 is a marvelous camera and I am thrilled. I wanted (and needed) superior low-light performance and that's what I got, among other things.
Back on-thread, reacting to the OP's question, if I had answered all the questions suggested about how a new camera might serve to resolve current problems, I'd probably plump for a D7000, essentially replacing the D70 (a camera I love and the one with which I started my DSLR adventure), which last can still take perfectly fine images when used within its capabilities. It's just that those capabilities are substantially constrained compared to those of the D90 or, even more so, the D7000. Family photos are often taken inside and flash is not always the most beautiful solution to a tender moment.