How much coolant came out when you replaced the thermostat? It could be that there were still some air bubbles left in the system after doing that and you just haven't put enough back in and/or bled the air out of the system properly.
does it overheat when the car is moving? or only when at idle? if u overheat only at idle, ur water pump is messed up... i would try and bleed the system first and see if tht works first.. just take the radiator cap off and run the car until the coolant starts flowing
Actually if the water pump is gone it will over-heat regardless, and it will over-heat really really quickly.
If you only over-heat at Idle, and cools down when you start moving again, it means that it is 100% sure it is the Fans. Either the Fuse, Relay, Fan Motor or the Fan Switch... the last being the most common one.

