There was a story on Tom's about a month ago on MSI "purposefully" altering their "regulators" so the Card would perform better than stock, at a risk of killing the card/computer it was in. It was specifically with the 670's Power Edition. I have a gigabyte card as well, hd 7950, its been running great, gigabyte is trustworthy in my book, not a bad experience yet (knock on wood).
EDIT: I'm not sure if they fixed it or not, but if your leaning toward MSI it would be worth a read.