A good lens is a lasting investment that will outlive generations of camera bodies. But you should be aware of these general considerations when buying a lens.

1. The greater the zoom range the lower the resolution, the greater the distortions and the smaller the maximum aperture. This is an unavoidable law of lenses. Thus a prime lens is capable of the highest quality, moderate zoom lenses are capable of intermediate quality while the long, so-called super-zoom, lenses have the lowest quality. It is a trade-off, convenience vs image quality that will depend on your needs and circumstances.
2. High resolution camera bodies will reveal the shortcomings in your lenses. Your Nikon D70 has a 6 Mp sensor and this low resolution sensor will not reveal the shortcomings of a long zoom lens like the 18-200. But when you upgrade to a new, high resolution body you will find the shortcomings of the lens become very obvious. At the same time you will have become a more experienced photographer who expects more from his equipment.

Many photographers happily make this compromise while others demand the highest possible sharpness and will only use prime lenses. It is your choice but you need to be aware that upgrading to a modern, high quality body will make the compromise more obvious.