I know you prolly don't want to hear this, but RMAing a card as defective when in fact you damaged it (by OCing it) is both dishonest and unethical. "Do unto others ...."
The 'right' thing to do would be to call ATI, honestly tell 'em what you think happened, and ask if maybe they can repair it for you for less than a new one would cost.
Not intending to come across as the morality police here -- just honestly trying to help.
EDIT: I was typing my post as Dave was posting his, so I hadn't seen it. I wasn't intending to pile on, JE78. So now you have an angel on one shoulder saying, "C'mon, you know the right thing to do," and a devil on the other shoulder saying, "C'mon, go for it! Screw ethics! They'll never know!" Quite the unenviable sitch! :laugh:
Good luck, mate.