Too windy and cloudy for me to have a play with IR at the moment, so I thought I should maybe keep abreast of all this modern technology. I have downloaded the instructions for SB600, SB800 and SB-900.
A quick scan through shows :- Quote from the SB-600 instruction manual section about Advanced Wireless Lighting ( page 60 ) -
" The SB-600 can be used as a remote flash unit only. " So you would need to buy an SU-4 Slave Controller for wireless control of multiple units.
Page 84 of the SB-800 manual suggests setting the unit to SU-4 mode in its custom settings. Effectively 'Commander' mode.
Page D39 of the SB-900 manual says SU-4 type wireless flash operations are available through the custom settings. That seems to be the same as the SB-800 which it replaces.
So there are differences other than power to justify the price difference between SB-600 and SB-800/900.
In fact reading all this if I was in a similar position and could work without Nikons Advanced Lighting system I would also consider a Metz 58AF-1N unit similar to Duncans. This unit is compatable with D-TTL, iTTL, iTTL-remote controller/slave. Link to Metz info on the 58AF-1N
http://www.metzflash.co.uk/pages/metz27.htm
SB-600 is similar UK price as the Metz but the
big difference is power.
SB-600 - Guide no 30
Metz58 - Guide no 58
Not sure if the Metz is available in your location though.
Don