its 540 core, and 490 on the memory, 980 effective. whoa whoa whoa, when did 6600GTs get >500 on the core?? That is unheard of. I can only get 410 on the core for my 6800GT. A screen shot would be nice.
Overclocking will set the new max the clock can go.
If you run 3dMark from an idle state and view your system info your card will be around 300mhz or so. Then run 3dMark and then check your sytem info and you will see your GPU is now at its max speed.