I've shot with a D50 for the last 4 years. It was my first DSLR. I finally moved to the D7000 this year, but shot a ton with the D50...all kinds of lenses. I always found reach to involve cost, so the 70-300 might be the only "reasonable" compromise that I could think of. You'll have 3rd party alternatives, and can sometimes find an used Simga or Tamron in the 500mm range for somewhat more than the 70-300, but less than the telephoto primes. I think Tamron may even have a rebate for their version of the 70-300 right now.
Agree a good fast prime would be easily achievable, the 35 1.8, or 50 1.8 AF-D for even less (which your D50 will auto-focus, unlike the D40/60/3000/5000). Wide angle is more affordable as well, if you are interested in honing that part of your repertoire. You can also consider the 18-105 VR at around 200 on the used market.