Remember that youc an't compare temps with different mb's, and you always have to adjust temps based on ambient case temps, nad program used to load the CPU.
Sandra can be weird, but the newer versions are much better.... Try MBM and see if the Temps match sandra's. Your "windows" temps will almost never match BIOS temps because the BIOS load is often neither Full nor Idle CPU cycles. The two readings should be different, and that isn't abnormal.
As far as how off your 8kha+ readings are? For XP chips, probably 5-10C lower than actual die temp. XP Chip PCB throws off socket-thermsitors moreso than do the T-bird PCBs.
Mike