For every fan that pumps air into the case, you ought to have another fan pumping air out. If you have an odd number of fans (i.e. one or three etc.), then give priority to the exhaust fans (i.e. three fans = one intake + two exhaust)
Ideally, you should at least one fan blowing air towards the CPUs heatsink/fan, and at least one fan near the CPUs heatsink/fan (usu. at the back) to extract the warm air.