I have an external probe that I will try to use for the next test. It is real tight in between the CPU and the water block, even more so with the XP's. I will look at the links above and try to figure out a better way.
I tested the on board prpobes agains the external probe the best I could and the on board was very close. it is hard to get a good temp reading off the CPU though. Even more so with the XP's with the little chips on the bottom and the CPU itself is narrower.
I burned up my T-Bird which I would rather do the tests with.

Always make sure you unplug the power supply before removing the HS/water block. I got in a hurry and forgot to and the system booted up (bumped the on button)and smoked the T-bird.