I want to point out that at least according to Bill Claff's numbers the D90 and D7000 will perform identically.
The D7000 will "drill down" deeper if you need to crop but utilizing the extra pixels in that way will likely just give you noise you don't want.
For $1000 you can get a 3rd party 70-200/2.8 and that buys you two more stops of effective high ISO sensitivity but it does it purely with the lens. Your cost is $500 per stop. In the grand scheme of things, and all the possible options, that is dirt cheap.
If you go the route of the D600 then my overly long and tortuous but I think necessary discussion above should illustrate why you are taking some steps forward and some steps backward, making it far less cost effective in terms of low light capability gained per dollar expended.
If Nikon were to come out with a "D7100" update tomorrow, that would presumably buy you another half stop or so for another $1400 (less whatever you can get for the D90). That is at the rate of up to $2800 per stop improvement. Very diminishing returns.
The D7000 offers many more features that need to be factored in- I'm just looking at it from a low light performance verses cost perspective.
A D600 + Nikkor 300/4 would run you about $3600 for a 1.5 stop improvement, or about $2400 per stop. Less if you can find a 3rd party equivalent. Strictly in terms of low light cost/benefit that is rather expensive too. A Sigma 300/2.8 gives you a 2.5 stop improvement at a total cost of about $5500, or about $2200 per stop.
That is why I said that in the grand scheme of things $500 a stop is dirt cheap . Even factoring in the princely price of a Nikkor 70-200 VR, that is "only" about $1200 per stop. A 70-200 is by far the best investment here.