They all the same!! Except for certain companies which install different coolers, but you usually pay more for these cards. But shop around, youll find awesome bargains. For example, here in south africa, you can get a Club3d 5870 for R3300, and an Asus 5870 costs R5200. The only difference = stickers!! Obviously you need to look out for the warranties offered by the different companies, as these often differ.
And then some companies also do overclocked versions of the cards, but again, these will cost a bit more.