Google for a program called ATITool 0.27.
You can use it to underclock your ATI card and also setup profiles as well. I use it to setup two profiles:
1. Low: Core = 175.50MHz, Memory = 279MHz
2. Safe Mode: Core = 450MHz, Memory = 550MHz
"Low" is used upon startup so when I simply surf the net or watch a movie the low speed is good enough to do the job.
"Safe Mode" is if I am playing a game and my video card is overheating, yet I still want to play. Setup just in case.
ATITool allows you to automatically switch between "low" and default speeds when it detects a 3D program (like a game) being started, and end it has been closed. 3D detection is not perfect though because it detects PowerDVD Player and Windows Media Player Classic as "3D" applications and it sets my X1900XT to default speed. However, you can setup an exclusion list that will not automatically cause the default speed profile to be used.
WARNING: The following option is not fully implemented yet.
Lastly, ATITool allows you to change the various voltages. While there are options to "save" voltage settings to a specific profile, the voltages do not automatically load with the profile. Therefore if you lower your voltages for your "low" profile, and you have automatic 3D detection enabled, as soon as you launch a game your system will crash!!
You must first manually increase the voltages back to default then launch the game. You can also create a "standard" profile to save voltage settings so that you can simply load the default voltages.
For my "LOW" profile I set VDDC t0 1.200v, and everything else to bare minimum.