This
I have a 7970 (msi lightning) , one of the best 7970's you can get and there's bugger all i can do to get it over 650.
My 7950's do 670-680 easy, and that's not even pushing them (I can get over 700 but choose not to due to extra heat/power).
Sounds like you took the worst 7970 out there. As I said, I've got a batch of 12 Sapphire 7970's that ALL do 750 easily, most of them more.