Here's my personal opinion:
Successful driving in snow is about 25% car, 25% tires and 50% driver. So you have to take care of half the problem yourself!
As for the car, as a RWD vehicle, no doubt with all the traction control goodies, and with FOUR snow tires mounted, you should do pretty well unless you try to do two VERBOTEN things with your BMW
1. Drive in snow that is higher than your car's ground clearance (you'll hang up)
2. Drive on ice (there is not a car in the world, even an 8 wheel drive, or an Army TANK, that can maneuver well on ice.
If you try to drive on snow with those run-flats you get on most BMWs, you'll kill yourself.