As mentioned by flipfreak, SB600's cannot be used as commander, and as suggested by ZerocoolAstra, those are what you can get if you need a more powerful commander. But do you? Unless you shoot outdoors CLS alot, I find the built-in commander on my D300 to be sufficient for indoor CLS use most of the time.
if your camera built in flash can be used as commander then use it..I use the D80 and it has this function, dont see why it shouldnt at higher end models.
u can select the built in flash to be in commander mode, with output (TTL or Manual) or without flash output (--). if u select off,
that means the built in flash fire out some preflashes ( too fast to detect with naked eye), for use as command signal to the off camera SB600s. this light from the built in flash is moments before the shutter opens, if i understand correctly, so it is not entered into the CCD.