1st Post, Venting question

First, I would like to give a HUGE thanx to everyone on the boards here. Being completely ignorant in the art of indoor horticulture, I came here looking for answers and tips. After combing the posts, many of my concerns have been laid to rest.

Now, for my NOOB question: My growing area is 4'w x 8'H x 2'd, though I plan on capping the height at 6' to maximize light concentration. I dropped the cash (now broke) for a 400w HPS light, which will be used throughout the growing process (post germination).

What kind of CFM will I need to insure proper ventilation, or what type of fan will I require? During veg, simply leaving the door open (sacrificing precious, life-giving, light) might be an option as I cetainly won't mind the dank aroma, but not during flowering. Attic venting would require activated charcoal in the solution. I don't think this lil' cpu fan is gonna' cut it :rolleyes:

think 4 x 8 x 2 = 64

therefore u need to expel 64 cubic feet of air an hour (i think)

However, something that small isnt going to help extract all that heat ur hps is going to make.

IMO, u want one that expells about 500 cubed feet an hour