I just got a new case with a front temp display. There is a sensor on the end of a wire. I don't know where to put it. Does it go between the heat sink and CPU? IDK.
personally, i would just use it as a case temp sensor. Because it wont even be close to the real temp of your hardware, i put mine between the hsf and cpu and it read only about 70F. So i would just mount it somewhere inside your case.
Also since the die of an Athlon 64 CPU covers pretty much the ENTIRE chip, I strongly discourage doing it because temp displays should NEVER be put on the die because temps get too high for something to be blocking heat transfer. Doesn't your mobo have an internal temp sensor that you can use with a program like motherboard monitor? I don't even use my case's CPU temp sensor.