The clocks go up 30Hz, and memory goes up slightly on the second. 3rd has 50Hz overclock but no memory clock.
The 7950 overclocks easily so tiny factory overlooks like those are kinda pointless. Id get the cheapest one, I prefer sapphire anyway, and its at newegg. Very good RMA experience with them as well. Lots of people like the twin frozer cards. I personally don't care for gigabyte.
They are all quality brands though, so it doesn't really matter which