AMD has a artificial limit on the pixel clock at 330 MHz. Download and run the Pixel Clock Patcher from here: http://www.monitortests.com/forum/Thread-AMD-ATI-Pixel-Clock-Patcher
Even though you just removed the *limit* CCC will not let you enter a resolution above this, HOWEVER you can regedit it to work very easily!
Make a custom res at a normal resolution that has a pixel clock under the *limit* i.e. 1920x1080 60Hz, CVT or CVT Reduced Blank.
Restart your PC, then open up regedit, calculator and CCC.
Go to CCC Custom Res tab and select the resolution you just made, then bump up the refresh rate (It will start making a 'new' resolution when you do this). i.e. 1920x1080 60Hz >> 1920x1080 75Hz
Next open regedit, goto the required reg location as shown in the System > Hardware tab in the new UI for CCC:
Once you go here navigate to the folder in the registry location above named DAL2_DATA..../Common/
Now select the folder which has the key 'ModeTimingOverrides_' DVI or HDMI whatever it is
Select this ModeTimingOverrides REG_BINARY and then save it out of regedit.
Open it in an editor of your choice.
Look for the three bytes which indicate the pixel clock you have currently set, now go to calculator and set it to programmer and punch in the pixel clock you got from CCC when you changed the refresh rate to 75Hz.
Now all you have to do is go a few addresses earlier and look for the initial refresh rate (for my example it is 73Hz or 49h) and change it to your new refresh rate!
Save the reg key as a new file so you have a backup, also make sure you are not modding the currently used resolution, just in case it doesn't work when you reboot.
Run the new reg key and overwrite the old key.
Restart, select the new resolution from WINDOWS