In a small to mediium size studio, the optical (Infrared: IR) CLS signalling works well, as the longer wavelength pulses easily bounce off walls, floors, ceilings, etc. I have had them bounce around corners into adjacent rooms, and trigger remote units. With a little practice at estimating bounce angles, one can do some really interesting things.
I agree: Don't spend a lot of money on any of the Radio Frequency (RF) units until you have explored the performance of your SB900 and SB600s in the intended environment. Nikon CLS technology in its native IR mode is a very powerful, versatile tool. I suspect you will be surprised at how reliable it is in small to medium sized environments.
Concerning additional units, I would suggest the SB600, for three reasons:
1) You already have two and are familiar with their setup.
2) The SB600s are a bit more powerful than the new SB700.
3) Locally, the SB600 sells for $225, while the new SB700 sells for $330.
With four remote SB600s, and a few light modifiers (soft boxes, reflectors, etc.) you can cover an enormous range of portraiture.
HBB in Phoenix, Arizona Nikonian Team Member
Photography is a journey with no conceivable destination.