hi.. i have an xfx 9800 gt with the same problem. even on a cool night like tonight, gpuz shows temps by the driver at 65C and the sensor chip at 58C. my cpu is about 38 C.
check to see if your gpu cooler has a small square plate on the back with four tiny screws holding it in place. one of the screws on mine is not flush like the others so its my prime suspect right now. also my cooler did not have any thermal pads for the ram for some reason.
xfx just sent a replacement cooler but its for a slightly diff model 9800gt
.. so ill have to wait some more to find out why.