top fans usually exhaust--as hot air naturally rises the fans exhaust it out
+1. And I should add that I am not a fan of side fans. My tests showed GAINS in temp for everything except the northbridge (The northbridge is right by the side fan). This was with all fans on high, including the side fan. I believe that the side fan should be a LOW flow fan! the side fan's purpose is just to keep the flow close to the mobo, I think. I have a low flow fan in the mail, and will run some more tests to be sure. You can run some tests with and without the side fan for yourself if you want. If you have a low flow fan, put that one on the side.