Not all cards are the same, and it depends how lucky you are. the older ones are easy to unlock, newer models, even though identical, may have problems. Remember that only the 2 GB model is unlockable. When the first 69xx GPUs were created, they had a high clockable rate, meaning they could all run at 6970 clocks, but couldn't be as AMD wanted something cheaper, thus 6970s were rebranded as 6950s and were deprived of one 8-pin PCIe and given a second 6-pin PCIe. Newer models may not be unlockable, so get a brand that is not heavily bought, and get reference card, not an MSI TwinFrozr. Basicly its overclocking, some CPUs overclock beautifully to 4 GHz, others fail to reach 3 GHz.
http/www.techpowerup.com/articles/overclocking/vidcard/159 has statistics on unlock failures/successes