We have very little real-world examples from D600 as of yet, but in DX crop mode, I would absolutely *not* expect it to "blow the socks off" the top DX camera (D7000).
Read the DxO numbers, it puts D7000 pretty much identical to D800 in DX crop mode, which means the higher resolution should put D7000 at a slight advantage over D600 in DX mode, which puts it as a slight but certainly noticeable step up from D90.
Anyway, it really doesn't make any sense to buy an FX camera to use as a DX camera just because Nikon hasn't made the DX camera you want. The main advantage and liability of FX has always been the bigger sensor. If you use the camera in DX mode, you lose the advantage yet carry some of the liability (cost).
Despite all the hype of the new bodies, the D90 remains an excellent camera by today's standards, unless your main use is very high ISO (> 1600 or so) or movies.