> You have improved my work considerably in a relatively short time
That's one reason I do this!
> As you have been a top flight photographer for sometime can you remember when the D50 was state of the art?
Yes I do: I started on an Exakta VXIIb, and have used Nikons such as the F2A and FM2n. But perhaps more relevant is that I actually still use a D100, which has the same sensor as the D50.
> Were the photographs then an improvement on what had gone before?
No, not really. The biggest difference is the greatly improved anti-aliasing filters used on newer cameras - that includes the D7000 and D90, D300 and probably D3100. This blurs the image less, meaning less requirement for sharpening. But for the most part, when used well, it's surprisingly hard to tell the newer from the older cameras. A D50/40/100/3000 etc is actually at least as capable as any Nikon/Canon/Minolta/etc up through about 1995 or so, and often much more so. That means that any of the great images from the 60s, 70s or 80s could have been shot on your D50 - and at least as well.
What's really different is in the boundaries of where a good shot can be accomplished. For your trains in Stockport, I'd say that there's only a difference in how big I could print the result: with a D50 or D100, it probably requires being quite careful to make a quality 24x36" print, whereas a 12mp body can make that same print fairly easily. Ditto for the shots of the runners in the park. But having the luxury of twisting the ISO up to 1600 or higher with impunity means that I can shoot sports under the lights, or match an outdoor exposure through the windows - but with a flash indoors. And I can keep a moving subject in focus even though I might be in a jeep that's bouncing around on a dirt road. With a D50 or D100 I wouldn't even take the camera out in some of those situations - even with high-capability lenses.
> I think I can swing the D700
Before you spend that much money, consider one of the DX bodies - which are considerably less money, and the D7000 and D300s are quite close to the D700 in capability. Check out my gallery; see if you can identify which ones came from which cameras and lenses. For the most part, it's really pretty hard.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!