its all personal preference,personally i would use 4 600's if your going to run 18 buckets eventualy and stager them.i would do 2 rows of 2.but not have them side by side.lets say ,starting on the left row u would have your lamp.3 1/2 feet down the ducting is your second lamp.on the second row i would start were your first row lamps ducting started then you would do the same with your fourth lamp. basically you wouldnt have your lights right next 2 each other in there rows. use y splitters at both ends of your lamps and use one large fan to pull the air from all four lights,damn i wish i had time to draw u a pic but work is calling my name.