From kernel 2.6.35, there is a support to the Ati PowerPlay Technology. Regulating the clocks of your ATI within linux (with the open source driver) is now possible. The interface is a sysfs interface, i.e. you must write some string in some files to change settings on the fly.
It’s described in the Archlinux wiki, at this page:
A graphical interface should be useful. I developed a small perl script using gtk-perl to change powerplay settings. I named it PowerPlaySwitcher. It’s a very simple utility. You can download it at these urls: