Sapphire and xfx I have heard they are both good. I would go with the cheaper of the two. Hopefully There are more 7870s on sale tommorow as I want to buy a 7870 myself.
It's usually just preference, but as you plan to overclock, consider the HIS IceQ, or other brands that don't have reference coolers. Gigabyte's take on the 7870 has its shroud rattling for a lot of the people who bought one, but they say you can just put some tape on the shroud or something.
I can voche for the quality of XFX. There cards are well made and they even replaced mine no questions asked when I busted it in shipping. Not to mention there shrouds are beautiful, all chromed out and shiny.
both Sapphire nd XFX are good companies. But Sapphire only manfucture AMD GPUs and their pricing more pocket friendly in compare to XFX which is more of a over priced premiur product