If weight is a concern for you, you might also want to consider the shooting circumstances. In certain situations, shooting handheld with a 500m may not be possible (blurring due to handshake) and you'll need a tripod if not at least a monopod. In such cases, even with a lighter camera and lens combination, you will still be required to carry around other complimenting equipment, this adds on to the overall weight. The D90 sports a 70-300mm VR lens, which means its going to be a little bit more forgiving with regards to handshakes.
Perhaps you could let us know what area of photography you're intending to get into and we can give you better suggestions. Some people use long lens for sports, plane spotting and birding but the circumstances that these lenses are used in varies quite a bit.