The title pretty much says it all. I have a gigabyte windforce radeon hd 7950 that has a clock speed of 1000mhz. When I'm not running a demanding game the clocks on both the gpu and memory decrease substantially, which is fine (gpu clock = 300mhz, memory clock = 150mhz).
I am using Sony Vegas Pro 12 to render my videos, and they included a nice rendering option that allows you to used your gpu to help speed up rendering times (usually by a lot too!). While rendering (using OpenCL that I know my gpu is compatible with) my clocks still stay at idle speeds and don't raise to what they usually do playing a game.