Using the D800 in DX mode will give about 15.5Mp so slightly more resolution than D90. However, the high ISO noise is substantially less and the dynamic range is more...so it will be a noticeable improvement.
Using the D600 in DX mode will give about 10Mp, so less resolution but with same ISO and dynamic range advantages as D800. the controls and menu system on D600 is an evolution of the D90 design so you will make the switch more easily but if you want to routinely use DX lenses then the D800 is a better choice.
It really is a bit of a waste to throw all those pixels away by shooting in DX so either make a commitment to buying some nice wide FX glass or stay with a DX camera such as D7100 which gives you 24MP on DX and a decent high ISO noise advantage over the D90.