cpu temp software which to trust HELP

The temp software that comes with my asus board and speedfan shows my temps at idle 28c and 100% load at 52c
but TAT and others read 38c at idle and 100% load at 70 to 80, at 3.2 OC but I mostly keep my cpu at 3.0 and would like to play with overclocking more bit I don't want to burn up my system so how do I know which to trust please help
ambiant 70f keep house cool reset heat sink but didn't change anything with the zalman 9700 it should run cooler and
my case is a antec 900 so it is well ventilated and at idle my bios reads 38c but in bios thats only at idle so what to do

Q6600
asus p5k
zulman 9700
4gig 6400
300 hdd
antec 900
2 ati 3870 crossfire
psu coomaster 650w

TAT seams to be accurate for E6x00 so should be for Q6600.

You forgot to indicate ambient temp. I have a E6400 @ 3.2 GHz. my temp is approx 32 C (ambient = 64 F - Hot wife likes cool house.
with ambient at 77 F, Mid summer, my temps are closer to 42 C. I have the Zalman 9500.

I would download coretemp and compare, But I think for the E/Q6xoo tat is pretty good.

On my system, Tat, speedfan ver 4.33, and Coretemp 0.97 all read within 1 to 2 C of each other.

The most accurate will be the temperature displayed in the BIOS.

With the latest BIOS update for your motherboard Core Temp should display the same temperature as the BIOS hardware monitor utility.