White residue is usually coolant. A block test can't hurt and is pretty cheap compared to a head gasket job so you want to be absolutely sure, though I hate to say it doesn't sound good.
Your coolant is going into the combustion chamber and being burned with the fuel, hence, the white residue it leaves on your plugs. The coolant then exits through the exhaust system. It may or may not ruin one or more of your o2 sensors, so if you get a check engine, its more than likely because of this. Your oil is probably milky from being mixed with coolant as well. I wouldn't drive the car, you risk warping a head or cracking one, or doing some other serious engine damage. I don't think your block is cracked, yet.So where is my coolant going?