I use the Overdrive tab in the drivers to watch the temps. I can't speak to the accuracy of it, but FWIW that seems about as accurate as you're going to get without breaking out a thermal probe. I just don't care THAT much about the accuracy of it.
Since ATITool runs in a window you can have the driver properties open next to it to watch the results. I am sure there is some variance between the actual and what it displays but that is the nature of the beast.
To be clear here, I am NOT advocating flashing the BIOS before you test the limits of the card AND the memory. AFTER, and only after you check both out to ensure stability and proper operation do I recommend flashing up to a PE BIOS. My card works at those levels without any problems and I just didn't want to have to d*ck with upgrading Rivatuner every time a new version comes out. Drivers are enough without adding extras in for "fun".