THERE is usually no difference between cards of the same model ie : 4850. The difference are only the sticker, the warranty, and sometimes slightly different clockspeeds.
If you look, you may come across a card with a different heatsink than the rest of the cards. These are generally better than the others as the cards stay cool. Examples are the Saphire ICE Q or whatever they call it.
Bottlom line, they are all the same.