before you go spending more money on parts, figure out if you are actually over heating or if the gauge is wrong. always when you chage thermostats,you should also change the temperture sending unit. It might not always be needed, however, it rules out the possibility.
check the lower hose to see if it is as hot as the top. (becareful, if it is overheating, touch anything for too long, and you'll burn yourself)
if the bottom one is cooler than the top, then its the thermostat. check for any leaks. If the water pump is going bad, it will start to leak.
there are lots of reasons why a car can over heat. so start with the cheap ones and work your way up.