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.