This is an interesting question, and I could write probably too much on it! I can give a few views based on experience.
Essentially, I think that the D300s represents excellent image quality.
First thing, I have a D90 (same pixels, similar sensor). I think it's still great and use it interchangeably with my 16MP D7000. True, the D7000 can give slightly better definition, but you have to print really large to notice it. In fact, I find my 6MP D40 quite satisfactory.
I think my 16-85 lens is one of the best I've seen (nearer the WA end of its focal lengths) and has superior performance to many fixed focal length lenses.
In fact, I used this combination in a very low light (gallery environment) recently and had really excellent results.
But I have always planned to go full frame and bought a D600 for its reasonably compact size, FX frame and good low light handling. 24MP is great definition, and you can see from the test on this site that it can comfortably out-resolve the 24MP APSC cameras.
Unfortunately, my D600 (and a replacement) had the oil problem. Oil on the sensor is one thing, but when you think about it, it's also being sprayed around the mirrorbox too and doing who knows what harm. My local store (the prominent specialist retailer in Canberra) treated me very well and now I have a D800. Not quite what I was after originally, but it has its own advantages and I'm very happy with it. Very pleasant and portable with the 50/1.8G. But, oil aside, I thought that the D600 had truly excellent image quality and handled low light very well.
If you want super detail, FX is the way to go, but you would need really large prints to see the difference. You would also need very carefully selected lenses to see the benefit. What is easy to see on your 27 inch screen at 100% can be irrelevant even in a fairly large print. Just depends what you want to use it for.
So - we don't know what the "D400" would be like or even if one is on the horizon. The D800 represents the upgrade from your sort of body type (the D600 is closer to the D7000 in body quality). The D800 could also initially use your 16-85 and give higher pixel density while wou acquire new lenses.
But your current camera is excellent. I've probably gone a bit too far in my purchases!
Nikon DSLRs, film cameras from Leica to Linhof