Ok heres my first grow setup I'm partitioning off the back side of half of my walk i in closet(8ftx5ftx8ft) I just went today to lowes and purchased these huge insolation panels called R-MATTE to make the false wall These things are great, no need for plywood,or any sawing, and they are strong to, about a inch thick and 11 bucks for a 8ftx4ft panel...anybody building a grow box should def check this stuff out in the insulation section at lowes....they have different thinkness ones so make sure you find these thick strong ones..............but back to ventilation question, I think I'm going to purchase a 400hps with air coolable reflector and 2 6'' 250cfm inline duct fans from htg supply. 1 fan will be to cool the light and the other will be paired with a DIY carbon filter for exhaust, will a passive intake at the bottom be sufficent or will I have to buy a third fan for intake?....oh yeah and keep in mind I will have a small regular fan inside for circulation and panda film inside and a door made of panda film secured by velcro...Do this idea seem good?

this setup sounds good. a fan pulling air in would certainly do alot for fresh circulation but i think you could get by without it, especally if you air cool your lamp seperately. good luck.