I've learnt some good lessons on cooling systems in my day job and i think the theories translate from diesel engines to petrol. The entire system should be balanced to maintain correct engine temperature for a wide range of conditions. The radiator, pump, thermostat, coolant and fan should all be considered part of the equation. Taily is right, the engine shouldn't be overcooled as it will suffer adverse effects. One more comment is to keep the coolant fresh and maintain a 50/50% mix of glycol and water. The glycol lowers the freeze point to around -36 Degrees C but it also raises the boil point in coolant, typically to around 106 Degrees C. This is further aided by a correctly fitted pressure cap with the best matched pressure rating which also helps raise the boil point.