I have just purchased a Gigabyte HD 7970 OC WindForce and happy with what it does. If I stick to its own parameters - Gigabyte's. Then I should be covered fully by their own warranty.
And food for thought - why doesn't the manufacture of any card clock it to the limit? Safety barrier springs to mind!
From what I have seen in various forums, the amount fps is not a great deal more when overclocking further! Not just my card but others in general.
What are the implications to its running life? Will it shorten it? I think if you have an old card that's passed its warranty, then yeah, overclock it to the max!
Or is it that in Amd's case, they under volt their PCB's in the first place. Hope you get what I am trying to say here, ha!