RE: overheats -possible head gasket ?
The problem could be a bad water pump, you should be able to determine if the seals have gone bad by visually inspecting it for leaks. Could be the drive belt to the pump has come off or is broken, again a visual inspection will reveal that. Thermostats do go bad, usually (but not always) displaying wide fluations on the temperature gauge that is noticable before they freeze up. The thermostat can be removed and tested, but most folks simply replace them once they have taken the trouble to remove them because they are relatively cheap. The problem could be a plugged up radiator, the car is the right age for that particularly issue if the owner has not routinely had the cooling system flushed out. A flow test at a radiator shop or dealer is necessary to check that. The electric motor that turns the cooling fan is controlled by a temperature sensitive relay that is served by a fused circuit. If either the relay has gone bad or the fuse has blown, the cooling fan will not turn. If so the engine will maintain temperature while driving on the open road at 40 to 50 mph, but will spike if you slow down or come to a stop. Start the engine cold and let it run until just starts to boil over. You should hear the cooling fan come on before that point. And yes it could be a blown head gasket or cracked head (you did let the engine cool down completely before pouring coolant back into the radiator, right?). With the engine cold and the radiator topped off, start the engine with the radiator cap off. Smelly exhaust gases should start bubbling back up through the neck of the radiator almost immediately.
If you did not crack the head by adding cold coolant to a hot engine, my guess would be the radiator or the pump drive belt if no other outward symptoms are apparent.