7970 Ghz Edition are usually Locked.
They can't be Overclocked much.
But 7970 can be Overclocked higher than Ghz Ed.
This is an over-generalization. It depends on the specific model... some 7970s are voltage locked and so are some GHz edition cards. Also, which card "overclocks much higher" has everything to do with luck of the draw and nothing really to do with rather or not you get a GHz edition or a regular edition 7970.
The 7970 DD BE originally was not voltage locked, but at some point midway through this past year they altered the BIOS to make it voltage locked. I know that the 7970 DD BE GHz edition was voltage unlocked as of a month or two ago, and I assume it still is, but I don't own one so don't know for sure.