The Inq wrote a post about the difference between AMD/ATI's own graphics cards and the ones from its partner companies, also using AMD graphics chips.
When you decide to buy a graphics card, the choice is not only Nvidia versus AMD, it's also often from which vendor to buy the card, after having chosen a given GPU model number. But given the same GPU chip type, is there really a big performance difference to be expected - if any - by buying, say a Gainward branded card versus a Connect3D one, just to give two examples? Of course, one card manufacturer can choose to release a DVI only version while removing the analogue connector, and other such basic decision about connectors and cooling, but other than that, what's the point of going card manufacturer A versus B?. Good question!