http://www.maxim-ic.com/getds.cfm?ln=en&pk=2578

http://pdfserv.maxim-ic.com/en/ds/MAX6657-MAX6659.pdf

Right, this chip isprobably oneofthe most accurate ways to get cpu temp measurements, as thesensor is inside the cpu. this chip will read the temp from it.

AFAIK, no calibration is needed, and any measurements from it can be thought of as exact within the chips accuracy *** (+-1°c) instead of *relative* measurements which can only be compared inthesame system, liek with thermistor sensors.

*** - might only be comparable for the same cpu though.

Quite simple circuit to make. Requires interfacing with smbus, which can be done on any recent stick of ram or onmobo headers, or abotu 50 places ona mobo.

Next it requires connecting to the cpu - easyest to cut the traces on the mobo for the cpu pins that are needed (unless they are not connected already..) and solder some wires going to this.

Samples from maxim are free, so I may have another go at making one. The first one I b0rked - the chips are REALLY small.