Thom is correct in pointing out the minimum focal ratios required to deliver the sharpest possible image at each common sensor density.
I believe I am correct in pointing out that even if you are not getting the sharpest possible image because you shoot narrower apertures, that does not mean that the image delivered is any less sharp than the same image shot with a lower density sensor.
You have the same issue today with your D300s. If you shoot that camera at f/22 you are getting a "soft" image, relative to what the camera would deliver at f/8, and that f/22 image probably has little or no additional detail than what you could get with a 4-6 mpx camera. Yet you do this without thinking about it.
The same would be true if you upgrade to a 24 mpx DX camera. That camera, at f/22, will probably deliver about the same amount of detail as your D300, and also about the same amount of detail you could get with a D2h (4mpx), D70 (6mpx), or a D700 shot in DX mode (5.2mpx).