Depends on the temp sensor that's being monitored. The temp sensors on a motherboard are a certain distance from a heat source and they are calibrated based on that distance. Of course, this can be largely influenced by the ambient temp, so yes, this is not the most accurate. However, motherboards can also read the on-die temps if the CPU has a built in temp sensor. I believe that all current CPU's have them. How accurate they are, I don't know. You can buy a temp sensor that has a probe and stick it close to the CPU's die. If it has a heatspreader like the current crop of CPU's do, you simply attach it to the side of the heatspreader with a thermally conductive epoxy, like Arctic Silver Epoxy. You don't want to pop off the heatspreader as Intel and AMD have them pretty firmly glued on, and that has the tendancy to take part of the CPU die with it. There's nothing like having an $500 CPU go down the toilet because of this, as I've heard many horror stories. I've fried a few CPU's in my time, but nothing that expensive.
