Flash Advice for d90

I purchased a D90 a year ago with a 16-85 VR lens. I have been slowly adding more lens and equipment as I learn more about the camera, about photography, and after I have taken a lot of picutres.

The 16-85 lens, when zoomed indoors, casts a shadow when the on-camera, built-in flash is used. I have faster glass (35 1.8, 50 1.8), but I think that I should have purchased an external flash eariler.

I keep thinking Nikon will have a new flash to replace the SB600. I believe the d90's own flash can act as commander and the sb600 can be off camera, hand-held and wireless, but I can be wrong. Is this a correct assumption?

Should I wait a bit? I do not want to spend the money for the SB900. Are there alternatives? Is a used SB800 an option?

Thanks in advance for any advice. (I tried to look at past threads but did not see any discussion that helped.)