This works with CrossFireX, you will not need anything other than Catalyst Control Center to overclock after following the guide. Only the GPU voltage is changed, memory stays at HD 6950 defaults. You will still have power control options too.

What this guide does:

1. Flashes a modded Asus HD 6950 bios with a GPU voltage of 1.175, unlocks to 1536 unified shaders, default GPU overclock of 810MHz

2. Raises CCC limits to 1620/2500

Here's the step by step guide:

810 MHz Factory GPU Overclock at stock HD 6970 voltage ( 1.100V + 0.075V )
1250 MHz DDR at stock HD 6950 voltage and timings
1620 MHz GPU Catalyst Control Center Overdrive limits
2500 MHz DDR Catalyst Control Center Overdrive limits
1536 unified shaders

Do not use or type the quotation marks

1. With your computer off make sure the bios switch is set to the "1" position, the position furthest from the CrossFireX bridge connections

2. Download this file, right-click it and select "Extract All..." to "C:\" or to wherever your Windows root directory is

3. If you have CrossFireX, disable it now

4. Click the Windows Start Menu button

5. Type "cmd" and press CTRL+Shift+Enter

6. A black window will appear, this is the Windows command prompt

7. Type "cd \hd6950flash" in the command prompt and press Enter

8. Type "atiwinflash -unlockrom 0" in the command prompt and press Enter

9. A progress bar will appear and disappear

10. Type "atiwinflash -f -p 0 bios.bin" in the command prompt and press Enter

11. A progress bar will appear and disappear

12. A window verifying the bios flash will appear, click "OK"

13. A window asking to reboot will appear
13a. If you have an HD 6950 CrossFireX setup click "No" and continue to step 14
13b. If you have a single HD 6950 click "Yes" and continue to step 20

14. Type "atiwinflash -unlockrom 1" in the command prompt and press Enter

15. A progress bar will appear and disappear

16. Type "atiwinflash -f -p 1 bios.bin" in the command prompt and press Enter

17. A progress bar will appear and disappear

18. A window verifying the bios flash will appear, click "OK"

19. A window asking to reboot will appear, click "Yes"

20. After the computer reboots you might see a window saying you need to install video card drivers, reboot again

21. Open the "C:\hd6950flash\SmartDoc674" folder

22. Launch the "setup" application to install Asus SmartDoctor, follow the on screen instructions

23. After the computer reboots, launch Asus SmartDoctor

24. Under Asus SmartDoctor's settings, check the "Overclocking range enhancement" option

25. Click "OK"

26. Exit Asus SmartDoctor

27. Uninstall Asus SmartDoctor

28. Launch Catalyst Control Center

You should now have a default GPU clock of 810 MHz and a default Memory clock of 1250 MHz. Your OverDrive limits should now be 1620 for the GPU and 2500 for the Memory. Your GPU voltage will now be 1.175V, the memory should stay at the HD 6950 factory voltage/timing defaults. You don't need any software other than Catalyst Control Center to overclock now.

I like to set my OverDrive to 880 GPU, 1250 Memory, and 12% Power Control.