I have changed the thermostat, and Waterpump. But it is still overheating. Someone said that it might be the head gasket.Will that cost me alot of money? Will that sovle the problem?

Before you tackle the head gaskets I would check a couple other things first:

• If you just replaced the thermostat and the water pump, that means that you've recently added coolant to the cooling system. I would check for air bubbles in the system by 'burping' it. In order to burb the system you need to open it up either at the radiator cap or at the bleeder valve. On a lot of cars you can find the bleeder valve by following the upper radiator hose to the thermostat. Slightly open the bleeder and squeeze the upper hose a few times to try and work out any air bubbles. You can also run the engine with the bleeder open or the cap off to try and circulate the coolant and allow any air to escape. Air bubbles in the system can cause overheating.
• Another thing you can check are your fans. Run your car for a while at idle to allow it to heat up. When it gets to the point where it starting to overheat check your cooling fans to see if they're running. If they're not, chances are that the coolant temperature sensor has gone bad which is fairly inexpensive and easy to fix.

If it isn't producing a white sweet smelling smoke - it probably isn't a head gasket. I'd look into the possibility of a clogged radiator or the cooling fans not coming on as required.