Heat dissipation is going to be your number one problem with HPS systems. 400w and 600w HPS will produce a considerable amount of heat that you will need to vent away from your plants. Having a air cooled hood will significantly reduce the amount of heat in your 3x3 area. Having good ventiliation is always a must.
Quoting the Marijuana indoor grow bible by Jorge Cervantes
"Under normal conditions, the ideal temps range for indoor growth is 72F - 76F. At night, the temps can drop 5 - 10 degrees with little noticeable effect on growth rate. Maintaining the proper, constant temps in the grow room promotes strong, even, healthy growth. Make sure plants are not too close to a heat source, like a ballast or heat vent, or they may dry out, maybe even get heat scorch.
Temps above 85F are not recommended even when using CO2 enrichment. Under the proper conditions, which are very demanding to maintain, higher temps step up metabolic activity and speed up growth. The warmer it is, the more water the air is able to hold. This moist air often restrains plant functions and decelerates growth rather than speeding it. Other complications and problems result from excess humidity and mositure condensation when the temps drops at night."
