Lots of white smoke coming out the exhaust, along with loss of power, indicates a leaking head gasket or intake manifold gasket. The white smoke is steam from coolant that is being sucked into the cylinders and vaporized.
Have the cooling system pressure checked. If the system will not hold pressure; a compression check on all the cylinders will determine whether or not it is a leaking head gasket. If the cylinder compression is low or uneven, there is probably a head gasket leak, or a cracked head or block. But if all the cylinders have good compression, and the plugs are wet with coolant; the leak is probably coming from a bad intake manifold gasket. In either of those cases, the engine will have to have the damaged parts removed and either reconditioned or replaced.
Incidentally, General Motors Dex Cool coolant has been found to damage manifold gaskets. A massive class action lawsuit over this issue was recently settled; and GM has agreed to pay a fixed amount for repairs for cars covered under this settlement.
It is normal for a vehicle to put some white smoke out the tailpipe when it is warming up. This will often be accompanied by a rich smell; particularly in cold weather. If you drive the car at least 5 to 10 miles, and it still smells rich, then there probably is a problem.
But it can be expected to smell rich during the first few minutes of running.
Other items which could create excess richness and loss of power when the car is warm are a sticking EGR valve, a defective air charge temperature sensor, a defective idle air control valve, or a defective throttle position sensor.