The D50, if memory serves me correctly, has a 6 MP sensor. The D7000 has a 16 MP sensor. That means the D7000 has lots more resolution, and that means you can crop a D7000 picture more and still retain enough to make a print when compared to a D50.
So with the same lens you will be able to make larger prints, or crop more while still getting the same size print as with the D50. Mechanically the D7000 has a lot more features and is just a more capable camera. So how will it perform with the same lens? Better, I believe. Everything where the D7000 will outperform the D50, though, has nothing to do with the lens - it has everything to do with a better, higher pixel sensor that is less noisy and has higher dynamic range.