In case you own an ATI graphics card supporting PowerPlay (dynamic clock scaling for the the graphics processing unit GPU), you can activate this feature in X.org. Open /etc/X11/xorg.conf and add (or enable) the DynamicClocks option in the Device section. Please notice that this feature will lead to crashes on some systems.
Section "Device"
[...]
Option "DynamicClocks" "on"
EndSection