dont use a fan for intake, just use both as exhaust and have a hole slightly larger than your exhaust as intake (active exhaust, passive intake.) Place the exhaust at the very top of your box and the intake at the bottom on the opposite side. By having the exhaust at the top it removes the hottest air first. And this way, if there are any holes or anything in your system they wont be letting skunky air out.

edit: being a computer expert, i might suggest some more powerful fans, check out sidewindercomputers.com and look at the panasonic panaflo series.
dietpepsi0219 Reviewed by dietpepsi0219 on . cabinet grow hi this is my 1st post here and im just after some tips with my set up. http://i79.photobucket.com/albums/j128/uksmoke/6179193A65UC258253M.jpg size (H)81, (W)80, (D)60cm. that will basicaly my base for the grow box,it will be coverd in light refective sheeting and because of the size i will be growing a lowryder cross probly masterlow under 1 200watt red spec envirolite. for veterlation i will be using two pc fans 120mm 44.8 CFM one as an exhaust and one for air intake. Rating: 5