>Looking for better quality images by using >the full frame sensor coupled with quality glass.
>My 18-105 kit lens is ok. I hesitate to put FX quality glass >on a DX sensor. I have not tried out the 24-70, so I can only >imagine that its effectiveness would not be realized / >maximized on a DX body.
While I have used my 24-70mm f/2.8G ED IF AF-S and 28-70mm f/2.8D ED IF AF-S lenses on DX bodies (Both lenses are optically superb on both DX and FX bodies), I found the Nikkor 17-55mm f/2.8G ED IF AF-S DX (Also optically superb) to have a much more useful range on a DX body. Aside from range, the 24-70mm produces excellent IQ regardless of the format (DX or FX). In fact the 24-70mm performs better on a DX body than an FX body because only the center area of the lens is being used.
>Why do people, in general, upgrade?
Assuming you mean transitioning to FX: To gain better high ISO performance.
>Many Nikonians have >multiple cameras and lenses, working in combination (I assume) >to achieve desired results in a variety of situations. I am >almost certain that the 70-200 on my D7000 will not get the >desired results of my grandson running around on a soccer >field, or running up and down a basketball court.
The D7000 with a 70-200mm is more than capable of capturing excellent images of both within the limits of the lens and the camera. For soccer, assuming daylight games, even on a DX body 200mm is on the short side. That said, as long as your subject is within a reasonable distance of your shooting location you should be very happy with the results. Given the advantage of the 1.5X crop factor, the D7000w/70-200mm will provide the Equivalent Field of View of a 105-300mm lens on an FX body. When shooting Basketball the limiting factor is the ambient light level in the gym. If you are very lucky, the gym will have an excellent lighting system, but the odds of that are next to nothing. The average gym has the ambient light level of a dungeon. This is where an FX body's high ISO capability is the tool to have. There are a few gyms so dark that a fast prime (f/1.4 or f/1.8) is required even on a D3 or D4. In the average gym, the D7000 w/50mm/85mm f/1.4 or f/1.8 should be able to hold it's own compared to an FX body w/f/2.8 lens.
>Perhaps my desire to upgrade is somewhat driven by emotion and the >feeling that a more dynamic piece of electronics at one's >fingertips can only help to achieve quality results.
Call it NAS! Your D7000 has more Dynamic Range than a D4, D3s, D3, or D700. So does the D7100.