I had a Nikon D50 for 5 years and it served me well. When I thought of upgrading, the two contenders were D7000 and D300S. But then I got a good deal on a used D300 ($715 for D300 with 12K shutter actuations and MB-D10 grip) and bought it yesterday.
I have 4 lenses - Tokina 11-16mm F/2.8, Tamron 17-50 F/2.8, Nikon 50mm F/1.8D and Sigma 70-300 F4-5.6 DG macro. I tried a few shots with D300 yesterday and couldn't click even a single half decent pic. I think the learning curve is going to be quite steep for somebody upgrading from a D50.