If you already are invested in the Nikon system, I'd recommend you stick with it. I moved from Canon film to Canon DSLR a couple of years ago and it was a natural progression. Of course the "I can use all of my old lenses" justification went out the window when L fever took over.

For all the "mine is better than yours" battles, what you do with it in your hands will outweigh almost all of the differences between brands.

If you were starting fresh, I'd recommend a Rebel XT or 30D at the lower price points (I think they're slightly better than the Nikon equivalents), then a D200 (clearly better if you want to cough up $400 more bucks), then the 5D, 1DsII. But at the D70/20D price point, it's neck and neck - if you've been with Nikon it's better to stay there.

There's always the "something new is coming" fear. I still have my "ancient" 10D. Guess what. I still takes great pictures, with perfectly clean 18x12 prints. The D70 is probably due for a refresh - dpreview.com's Nikon forum should have the latest rumor mill. But unless it's supposed to out in the next month just make the move and don't look back.

There are adapters to let almost any lens mount on any body, but you lose almost all automatic control and it's not a good thing to depend on, IMHO.