>... I think the learning curve is going to be >quite steep for somebody upgrading from a D50.
I'm surprised to hear this, because I upgraded from a D50 to a D300 and from day one my D300 photos looked good. I mostly shot aperture priority on the D50, and just set the D300 to aperture priority and started shooting with the default settings. There is a lot to learn on the D300 so I'm not claiming I was an instant expert on the camera. (I've had it more than four years, and I still wouldn't claim to be an expert.) But even though I was a novice with the camera when I got it, I didn't have any trouble getting shots that matched what I was getting with my D50.
You have some nice lenses. From your post I'm not sure why you're not getting good results.