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:
https://wiki.archlinux.org/index.php/ATI#Powersaving
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:
http://power-play-switcher.googlecode.com/svn/trunk/PowerPlaySwitcher-0.2.zip
http://power-play-switcher.googlecode.com/svn/trunk/PowerPlaySwitcher-0.1.zip