[Lethal]

You need to check that there actually is a thermal sensor there to begin with before you can check the temperature. I thought Ati tool can detect if you have a sensor, I don't know where to find a program that monitors graphics card sensors, but Motherboard monitor and Sisoft sandra can detect mobo sensors.
personally I just stick my hand against the lower side of the case and fell the heat of the side. Warm means you should be careful and invest in a exhaust blower maybe, and if its hot your screwed as you have probably done damage to your card for a mere ~10fps increase.