Comparison between D70, D70s, and D50 here

There are adapter rings between Nikon lenses and Canon cameras, but you lose the electrical connection between the lens and the body. All camera control of the lens goes away, i.e. aperture, autofocus, etc.

If you have a collection of *good* Nikon lenses, there is no reason to switch. If your lens collection is so, so, then the field is wide open. Nikon, however, makes a quality product.

As noted above, no time is a good time to buy a digital camera with the field evolving so quickly. I shot for years with a 6mp Canon 10D and felt no need for more pixels. I recently stepped up to a 12.8mp Canon 5D, but that was mainly so I could take advantage of my collection of full-frame lenses.