First, you have the top of the line camera which is quite more than you need to shoot anything you've described. You bought a Ferrari to drive to the store. You would have been better off getting a D300 or two and top quality glass. Your lenses will contribute much more to quality images than the camera.
SB900s are good, if you know how to use them and have appropriate light modification for them. If you insist on using speedlights, the master of using the Nikon system on location is Dave Black. Go look at:
www.daveblackphotography.com/workshop
Also check out
http://strobist.blogspot.com
Your problems with the speedlights will be recycle time and power (batteries die). Bigger than those, unless you modify them right, you'll get hard shadows that are unappealing and keep your models from looking their best.
Personally I'd take the SB900s back and go hit AlienBees.com for the best strobes for the money, along with soft boxes. If you need portable power, they have a good battery pack system as well.
Good luck.