400w HPS, checking out High Tech now ;DWhat size light?
I would go with a 6" and a fan that has at least 280 cfm. Get a 6" fan with a speed control.
7*2*4=56 cubic feet
You want to clean the air 5 times a minute.
56*5= 280 You need a fan with a minimum of 280 cfm.
I like high tech garden supply.
I have noticed a bunch of new, cheaper gear from Bloomerang around the internet. I know nothing of the brand except the 600 watt hps performs better than big name brand digi ballasts.