Having recently read one or two posts about fans something occured to me, should a case fan blow air into the case, or extract air from it? Would it make any difference in a normal household situation anyway?
My last PC had 2 fans, one sucking cold air into the case and one blowing the warm air out the back. My new one only has one fan and it sucks the air out of the case. If you have more than one case fan what you do not want is both of them sucking air into the case as it'll just heat up.
Generally, a front mounted fan sucks air in and a rear mounted fan blows air out. The important thing is to create an air flow across the motherboard, and which direction it flows is unimportant as long a it doesn't fight the cpu and psu fans.