I think it depends on what kind of shooting you do.
Also, I understand and generally agree with the thoughts above, but I do think there were some nice qualitative improvements in the body and the sensor between the D300s and the D200.
As for the D600, the big question there is how important to you is the handling of the camera. Go to a retail store and check out the D7000 or a D600 if they have one to get a feel for the way it is laid out. There is a lot more "press and spin" vs. turn a knob or flip a switch going on, and while I lived fine with my D90 that had very similar handling to the D7K/D600, I MUCH prefer my D300s.
Rent a D600 for the weekend and give it a spin. Recognize the impact on your lenses (and possibly your PC hardware and storage if it isn't fairly fresh...) from the bump it would give you. It is a heck of a camera for the money though.
Lastly, the D400 might be here by Spring, or it might never be here. Hard to tell at this point.
Here's the good news. None of these cameras are disappearing forever. Keep socking away a few bucks and then decide what you need. It might be glass! The biggest jumps I ever got were from better lenses...