i replaced the water pump on my 2006 grand prix and according to the gauge it still overheats. i plan on changing the thermostat tomorrow. if that doesnt work, what next?

check the radiator / hoses for blockage & see if heater core & hoses is ok.. how hot did it get in 5 minutes?

are you using distilled water & coolant in the system? tap water can leave alot of calcium deposits in the radiator. i stopped using tap water since '86 buick.

After draining cooling system you need to purge the air out. Not a bad idea to go ahead and change thermostat anyway. Then be sure to purge all the air out.

Replace all your coolant hoses -- as they get old they get soft - I had one that was collapsing as I drove and stopping the flow -- drove me crazy until I figured it out (also do the stuff Jason and aztec say above of course!)