Overclocking can't be considered as a defect if it runs normal operations.... The 7970 is the same as the ghz, but not guaranteed to run at higher clocks, and might have better fans and voltage management (.
The faulty ones are usually user error (bad psu, static fry, bios settings), but can also be board related (doa, or needing a vbios update)
Just look up 7970 vs 7970ghz, theres a toms article somewhere.
Remember that you will see more defects in forums because the functioning ones are not being called out (people don't go on forums to say how their cards are working just fine).