your gonna have to get your temps below 80 if you want healthy vigorous growing plants and good quality buds. i had a fan problem once and the temps were in the mid 80's for about 5 days and my plants really suffered for it. i consider over 80 to be too hot for mj. others have had luck with temps in the low 80's but if you can keep it in the mid seventy's or lower you will have much better results.

be careful pulling air from outside as others have suggested if you are in a cold climate as i am you could easily kill your plants with way to cold temps.

my intakes just take air from the house and pump it threw the grow then i vent threw the wall and outside the house on the third floor. i have enough air flow that the temps in my grow are always the same as the house, which ranges from 72-78 normally. you can get a 250cfm or better exhaust fan for as low as $50 (or lower) if you shop around.

i included a couple pics of what you get if you can keep the temps in the low-mid 70's. if you have any more questions about ventillation options feel free to ask me.

my main flower chamber has 400hps and 168watts of cfls and with 360cfms total for the 2 exhaust fans my temps never even get to 80. in my opinion a computer fan is useless to use for ventillation with a 400hps you need more power. hps puts out alot of heat!

i just started this thread yesterday, drop by anytime!
http://boards.cannabis.com/showthread.php?t=52326