I have the same card but the AIW version.
First, make sure that your fan is working. There have been numerous reports by other owners on the net that the 9800 fan is quite vulnerable to dying.
Second, make sure that there is no dust build up on the heatsink of the 9800 Pro.
Also are you OC? Because if you are you should make sure you have adequate case cooling at the minimum e.g. one fan blowing cool air inside and one or two fans blowing hot air out.
If you have soundcards and other PCI devices try to install those further away from the 9800 Pro because soundcards get hot too and if you one too close it just adds to the heat.
Turn off or turn down AA if you have it on high. It really stresses the GPU.
If your room temperature is really hot (above 24C) you should consider moving the PC to a cooler spot or opening some windows
in your room.
If all those "cheap' methods fail, just buy a VGA cooler. Zalman is a good brand.