I sold my 5970 a long time ago actually for a 6970 because of this. My framerates took a dive when cranking up AA, numbers were all over the place so games felt incredibly choppy, even though the average FPS was decent. It's something most reviews don't show.
The 6970 actually delivered a...
I tested my Sapphire HD4670 with GPU-Z and it is not able to read the fan RPM speed. Also, the shaderclock is wrong (349 c :P, obviously its not running that hot).