If you damage your 6990 while overclocking, AMD cannot help you nor replace the card, neither can Sapphire, and your warranty is NOT void if you take off the sticker, it would have said "Warranty Void if Removed" on the sticker.
|DAMAGES CAUSED BY USE OF YOUR AMD GRAPHICS PROCESSOR OUTSIDE OF SPECIFICATION OR IN EXCESS OF FACTORY SETTINGS ARE NOT COVERED UNDER YOUR AMD PRODUCT WARRANTY AND MAY NOT BE COVERED BY YOUR SYSTEM MANUFACTURERâ€™S WARRANTY.|
If you overclock passed factory specs without the switch i.e. using Afterburner then your warranty is void unless its RMA'ed with default clocks and a yellow sticker intact. Or just buy a Sapphire or XFX and remove the sticker.