I think SU-800 is a waste of money... It's just a commander right? D70s don't need that unless using I need to use different channels and/or groups... Even then, SB-800 will do the job... Then the rest maybe use SB-600... just that they don't come with a diffusion dome...
If you set your camera to ISO200, the GN is 30m. Meaning for distance 1m, you can use Aperture f/30 (aperture = GN / distance ==> Aperture = 30 / 1m = f/30).
For distance 21m, you need aperture 30/21m = aperture ~f/1.4, or at ASA1600 = aperture ~f/4.
Yah, not necessary using ASA1600 if you have f/1.4 lens .