My case has a temperature sensor too, and I just suspended it in between some wires to give me a reading of ambient case temperature. You can get GPU temperatures from the graphics card software and you can get CPU temperature from SpeedFan.
Whatever you end up doing, just don't stick the sensor on top of the GPU/CPU, because that will (obviously) break the contact between the chip and the heatsink. You could try slipping the sensor in between the fins of the heatsink, but only if you're really determined that there's a point to having the GPU temperature at the front of your case.