BMW 03 530

First buy a new radiator cap because the first time the coolant was released through the cap some debris may have prevented the safety pressure release from reseating ... It's essential that there is some pressure in the radiator because WATER CAN'T BOIL UNDER PRESSURE

I agree with Tom, replace all of the cooling system components, and do NOT overheat the car. If the car is overheating while you drive, pull over as soon as you can safely do so and turn the car off for a while and let it cool off. These engines are aluminum and can warp from overheating, so it is important to prevent it from overheating. BMWs have finicky cooling systems, so the best thing to do if you don't know the service history of the components is to replace them all (water pump, thermostat, all hoses, expansion tank and cap, and possibly the radiator) and fill the system with 50/50 coolant and distilled water.

Also, if you don't do your own work or don't know anybody who does, I recommend finding a reputable BMW independent mechanic. The dealer will rob you on prices. If you get your parts to bring to be installed, I recommend using eEuroparts.com or FCPEuro.com.