I think to sell the graphics card the company has to pass some nVidia tests but some brands are more prone to problems than others. I've heard of a few more faulty Galaxy 6800s than other brands. Also Gainward had a pretty shambolic time with their first 6800s (wrong BIOS included etc) so I would stay away from them. Leadtek, BFG (XFX in UK & Europe i think) make very good cards.
Well evga is a great company to buy cards from. They are actually coming out with new ultra's and gt's that have copper heatsinks instead of aluminum. BFG is also great but I think that their stock coolers are a bit different then the referance design. I think it was said that it is all one peice of alluminum instead of having seperate ones for the gpu and memory. But BFG overclocks their cores slightly and applies arctic siler at the factory. They also got a lifetime warrenty. EVGA has a thing called an upgrade plan so that if a newer card comes out within 3 months you are allowed to send your in and pay the difference between the cards to get a new one.