Use the nVidia montior option to watch your GPU temp
you should be able to just right click on your desktop and go to the nVidia Control Panel, then click on the thing that says Monitor Temperature levels
coretemp is just for the cpu, as far as I can tell w/ mine atleast
The bottom number in ATI Tool seems to be the card's temperature, and the top number seems to be the core temperature. So it looks like it's agreeing with the other two programs, if what I said about those two numbers in ATI Tool, is correct.