Off-Camera Flash

I have a Nikon D90 SLR and an SB600 flash unit. What Nikon flash unit is available to be fired off-camera with my D90? The SB800? The SB900? Will my D90 fire either of these without any additional equipment? What if I got two D800's? Would it fire both of these at once off-camera, or do I also have to buy a "whiz-jig" also? If I bought another SB600, what else would I have to buy to be able to fire both of these off-camera? I am thinking I will need dual off-camera flash to get into wedding photography. Your comments?

Find out if the D-60 supports the Nikon Creative Lighting System. If so the pop up flash will trigger the SB-600 as a slave. If not you will need an SB-800 or 900 as a master to trigger the 600 as a slave. The 600 will not function as a master in the Nikon system only as a slave.
The other option is to go with Pocket Wizards or one of the other radio trigger systems. The Nikon system works, but is finicky. Wizards are a more reliable way to go. More expensive too.
Being a sports photographer I don't shoot weddings so I can't comment on that aspect of your question.

The D90 does support CLS.

I have never seen a wedding photographer using multiple flashes. You are usually moving around a lot to follow the couple and that means being mobile. Having extra flashes on lighting stands is going to slow you down.

Based on my limited experience with weddings and off camera flash...it doesn't work out well. For posed shots sure, but the rest of the time you're moving, moving, moving.