It should only "underclocked" @500 in 2d mode and automatically switches to 750 when you start a game up, this is normal. I monitor this through everest with a G15 keyboard. To overclock it you can use the overdrive in the CCC after unlocking the padlock icon, just try it in 10 step increments.
Another way is to make a profile in CCC and edit the XML files it creates. The files are usually located in C:\users\<yourname>\AppData\Local\ATI\ACE\Profiles in Vista. In the XML files you have total control over core/mem and fan speeds on desktop and 3d modes.