Quote>I use my D300s and D200 with a two camera strap to take pictures of musicals.>Quote The D200 is dying.
Quote>So what to do? 3. Buy a D600 for $2100<Quote now only $1999 with 24-85 lens or used for under $1500.
Quote>I'd like to get better low light performance. Rumor has it that the "D400" will have ISO 6400. That's only one stop more than the D300s. That's also true of the D600, but I'm sure the D600 will have lower noise pictures at ISO 6400.>Quote Not sure about the "1 stop" but you will get better noise free images at more than twice the ISO of the D300 with the D600.
Quote> But if I get the D600, my favorite walk-around lens, the 18-200mm DX, which I use mostly when I'm not shooting musicals, means I'm in 10-megapixel crop mode. Quote Get an AF-S Nikkor 28-300 VR, it is a much better lens and FX.
>Or maybe I should consider one of the other new cams, like the D7000...Quote The D7000 will give you better images, especially at higher ISO. But, consider a used D700. It is the best match for your D300. What ever you decide on paper, got to a camera store and get "Hands On" first. You will have a difficult time getting good pictures if the camera is uncomfortable to hold and use.