Your answer is here at dpreview.com. Just scroll down to where it says ISO and you'll have your answer. The D70 hits quite a bit of the ISO numbers in between 200-1600 but I rarely switch from ISO 200. If it gets too dark I prefer using a tripod instead, but a higher ISO is good for action shots that can't be accomplished with a higher shutter speed or if you are shooting low lighting without a tripod. The D70 has little noise from 200-400 but of course 200 would be your choice for the least noise.
I also wouldn't use the auto ISO setting because the feature doesn't work as well as it should. It tends to select a higher ISO more often then I would like. Most photographers like to set the ISO manually anyway.