Well, the most accurate way to verify your temps is with a hardwired probe inserted into the heatsink block bewteen the block and the CPU wired to a readout.
Not many people do that
but that is what they did in the test I saw testing PC Probe I and another manufacture's board and their software. I can't remember which one unfortunatly.
Check the CD that came with your motherboard and see if it includes PC Probe, or PC Probe 2. Again, the ASUS site does not list it as a valid utility for your boad. Not sure if you could download the version of another P5P800 based board and use that, but might be worth a try. Or contact ASUS directly and ask about it........