Hmm... what p3 do you have? And what mb? Because if you have a diode-reading p3 board, why would you need the compu-nurse probe, which is inherently less accurate than the diode reading?
If you don't have a diode-reading mb, you can mount it like this:
TechnoNuts Thermal Sensor Placement
As far as the radeon, if you can mount it so that it touches the edge of the core(not between radeon heatsink and chip, but on the side) that would probably be optimal.
Mike