I have a Nikon D50, it is now my backup - I migrated to a D300 a couple of years ago. The Nikon 70-300mm f4.0-5.6 VRII is an excellent lens. A new choice, for about $100-150 less is the 55-300 mm f4.0-5.6 VRII which is DX only - but that is not a problem since the D50 is a DX body.
However, given that you are already shooting a 55-200, that would not be my first suggestion. The 55-200 has pretty good telephoto reach. My suggestion would be to get a wide to normal zoom something in the 18-55mm range. You can get the Nikon 18-55mm f3.5-5.6 VR for about $180 new, or as little as $99 if you are willing to get a Factory refurbished lens(by Nikon) The key difference is that the refurbished lens comes with a 90 day warranty while the new copy has a five year warranty. My 18-55 VR I bought as a refurbished copy for $99, which is usually on my D300. I have had it for about 2 years with no problems, but YMMV. I got the 18-55 non VR as the kit lens when I bought my D50 and still use it.
My alternative recommendation would be a fast prime lens. I would recommend the 35mm f1.8 G which costs about $190-200. It would be an excellent complement to your 55-200.