Per the last Intel C2d and quad data sheets I read, Intel specs the accuracy at 1 deg C.
These programs that read the CPU register that provides the raw data the temp is calculated from do not/should not/cannot/read the temp value constantly so there is some averaging over time but in general there is nothing more accurate than one of these programs if it is properly coded.
The bios reading is typically not using the coretemp method so it is an apples to oranges comparison. Coretemp can be trusted to be pretty damn accurate. Someday the bios engineers might wake up and use the same method.
Here is how coretemp and other programs that properly read the CPU core temp from the embedded Digital Temp Sensor (DTS) work and how if you are curious you can see the raw data yourself and calculate the temp.
Note: Intel has been constantly improving the DTS, on current CPUs there are actually many temp sensors embedded in each core in an attempt to cover the possible hot spots under different kinds of computing loads and internal circuitry presents the hottest to the MSR register that can be read to the outside world.
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