I was on a quest to get a custom screen on my G15 keyboard that had the info that I needed to see w/o the glitz.

I wanted: Voltages (VCore, 12, 5, and 3.3) Both core temps and the GPU temp. (threw in an analog clock for grins)

I used LCD Studio 2.0, Speedfan 4.32 build 17, and ATITool 0.26 and got what I wanted.

What all that unrelated stuff leads up to is that:
1) Speedfan comes close to TAT (within 1 degree) with temps on both cores.
2) ATI Tool gets the temps on the 8800s pretty well.
3) Without spending buku bucks I still have to load and run several tools to get the info I want on my G15 LCD but what the heck...