Here's another line of thinking. I will take the example of the D300 and D90, both of which I own and love. The D300 was introduced first (8/07) and is on the Nikon list of pro cameras. The D90 was introduced just a year later (8/08) and uses the same sensor as the D300. Does this mean that one is better than the other? Not necessarily in all parameters. In use, the D90 is much more like the D70 I started out with, which is to say somewhat more primitive than the D300. Flip side - it is an easier camera to learn. It is also lighter, more like your D5100 or D3000. But, the catch is, in the intervening year the good folk at Nikon had found ways to squeeze a bit more performance from that same sensor so that the D90 has a small but measurable advantage in high-iso image quality. Consequently, my D300 generally stays on the tripod and the D90 goes walking with me. I'll take them both to indoor shoots but the D300 will get to use the faster lens.
None of these arguments may make any difference to you - fair enough. It may be all about the controls or price or some other metric - again, fair enough. But there has been a steady improvement in the performance of sensors over the course of the last decade. Consider the release dates of the cameras mentioned in this thread: D70 1/04 D200 11/05 D300 8/07 D90 8/08 D5100 4/09 D3000 7/09 D300s 8/09 This list is culled from this article
The first four models and the very last offer you a significantly increased level of control (including a second, sub-command dial on the front of the body) over the 5100 and 3000. I would suspect that, while in most cases the D200 would be at least comparable to the D3000, sufficient progress in sensor implementation would have been made over the four year span between the introduction of the two bodies that in some not-so-extreme cases you very well might notice differences, and not to the advantage of the D200.
The question in my mind would be how best to position myself for the future within my budget. If all I were doing was tramping around taking pictures in broad daylight, any of those cameras would do. As my low light shooting increased, though, I would gravitate toward the newer bodies as a moth to flame. In that list, I think the D90 occupies a sweet spot at the intersection between price, control and image quality.