The D40 is better at high ISO than the D200.
This is how I'd rate things:
D3 / D700 - 2 stops better than the D200 / D80
D300 - 2/3rd to 1 stop better than the D200 / D80
D40 / D50 - 2/3rd of a stop better than the D200 / D40x / D60
D2X - 1/2 to 2/3rd of a stop worse than the D200, nearly 3 stops worse than the D3 / D700
You need to quantify what you mean by high ISO performance, no camera exists with clean ISO1600.
The D3 and D700 do have noise at ISO1600.
The D300 has more noise, and less detail. The D200 has more noise, and less detail. The D40 some noise, but good detail. The D200 has a chunkier unpleasant noise charactersist, the D300 is more fine grained, and less "clumpy", it has noise, but its more pleasant.
The real issue I think is how much detail is retained. The D3 and D700 retain much more detail than the others.
Ignore any "pie in the sky" claims about high ISO - regardless of camera. ALL cameras to date have noise at ISO1600, and you can start to see it from ISO800 even from the D700, I can see if from ISO320 from a D300. There is no noise free camera. The issue I think you should look at is detail retained, not noise (low noise is often at the expense of detail), and also the quality of the noise that remains.