I have a D50 and a D300, and in most circumstances you can't tell which camera took the photo. The D50 is a wonderful camera and I still use it a lot even though I have another camera most people would say is better. So, don't feel like it's much of a compromise to keep using your D50.
As to lenses, I also have the 55-300 that several others have recommended, and it works great with the D50. The image quality is excellent, you get more range than with a 70-300, it costs less, and it is smaller and lighter. The D50 and 55-300 make a great pair and if cost is a factor, as you say it is, the 55-300 is easy to recommend.