I don't shoot sports and I don't own a D70, so feel free to totally ignore my posts.
That said, I use a D100 for my studio shoots, concert photography and press conference events and I really hate it. The image quality is stunning, but using the camera each day is not pleasant. Having used the F5 for several years, I have gotten used to a certain build quality and a certain autofocus speed. Both are lacking on the D100.
From what I've heard, the D70 is a small step down from the D100. It's harder to grip because it smaller and you can't add the vertical grip. The viewfinder is a bit dimmer, and I would imagine that its a bit slower in the autofocus department as well.
I would never use a camera like that for shooting sports -especially if I enjoyed shooting with a pro camera like the D1. I would suggest you invest in a used D1x. They go for as low as $2,000 on this forum and on
www.sportsshooter.com.