I grow just fine in a 4'x5' with a 400MH/HPS convertible light. No need to overkill on the lights! Also, I think in that small a space, heat may be an issue with the big lights, even if their vented and cooled externally to the room. The 400's are cheaper to buy AND run, and replacement bulbs are cheaper as well....something to consider, as you will have to replace them occasionally.