You're asking us how much money you should spend. I'd go for a D4 + 600/4 VR and nothing less .
A D300s would buy you about 1.5 stops of low noise performance. In the grand scheme of things, $500 a stop is not bad . The returns beyond that are quite diminishing in terms of price/performance. It will also handle like your D200, except better (better AF, a few more buttons for things like LiveView. But the button arrangement is such that it will mate well with the D200 you use alongside it.
I wouldn't buy a D600 unless you go all the way and get FX glass for it. And that could cause some issues with shooting 2 bodies, DX and FX, unless you think things through and it may eliminate the option to fund it with your DX lenses.
I would buy a D200 to tide you over while waiting for the "D400". It could be a long wait without that 2nd body. It is really just a baseless rumor at this point. One million people wishing for that camera and talking about it as if it is around the corner will not, alone, make it happen .