錢,錢,錢.
D7100 is the next one you need but if you have money to spare.
Then A D800 is your move.(FX)
Don't go for the D600. Compare the spec before you make the call.
Good luck
D800 will be a good choice if money is not the issue.
FX camera have the crop mode. triple8's old 18-270mm can be reuse.
D800 is my next dream camera body.
Depend wat u need.... if as a professional photographer, of coz high end is better.... if for hobby, I suggest stick with ur D90, just upgrade ur lens will do