To use your D70/D70s' inbuilt flash as a commander and SB-800/SB-600 unit(s) as remote strobe(s) you simply need to do the following:
On your camera, go into MENU and then further into CUSTOM SETTINGS, find custom setting no. 19 (flash mode), select "commander mode", then further choose "TTL" and press OK. This might sound a bit complicated but it is really easy once you have done it. When you are done using inbuilt flash in commander mode do not forget to change back to TTL mode (go back to custom setting no. 19 and simply choose "TTL" instead of "Commander") as otherwise the flash will be firing only monitor pre-flashes.