Don't drive the car until you fix this...topping up the coolant doesn't count.
Bet you that your ET just popped...and there might be a reason why it popped other than it was just old...might be some other problem with cooling system.
Depending on your mileage, you could replace most of the cooling system (minus radiator) for $300 in parts, a few hours reading, and a few hours diying. That's your best solution.
If you want a less expensive solution, you'd need to read more so you can properly diagnose your failure.
Do not drive...you have a cooling system failure and it could lead to your needing thousands of dollars of repair or engine replacement...seriously.
I'm so depressed. No one wants to come see me. :(
Mangofy your E46!