You want to make the airflow go where you want it to go. You dont want a fan blowing hot air directly onto your memory or vice versa.
You should build a positive pressure case. Ideally you want 2-4 intake fans to blow inside the case, and 2 exhaust fans. If you have a fan on the cpu, aim the direction of airflow directly towards the exhaust fans, while the intake fans blow cold air onto the cpu heatsink and fan. My last computer had a 25cm case fan, 4 120mm intakes, and 2 120mm exhaust fans, as well as memory fans on the dimm modules. If you have enough pins on the mobo or psu, put as many case fans on it as possible, and make 2 of them exhaust. You just have to decide where you want the exhaust to shoot out of.