Thanks guys, that is pretty much how I expected things to work and that makes sense. And I understand that the Dx image is of lesser resolution, naturally. However, I don't know enough about sensors, etc to have any idea how this is likely to effect low ISO performance in DX vs FX mode. There may not be any difference at all.
When the D200 came out it was exactly what I needed and what I had hoped for. It is a superb camera. By all expectations the D400 (or whatever it might be called) would also be exactly what I need and what I'm hoping for at this stage of my amateur photography 'career'. And I might still wait for it a little longer. And while the D800 is almost certainly more camera than I can rationally justify, if it can use my current DX lenses to produce images of superior 'specifications' than my D200 then it remains high on my list of upcoming purchases.