nvidia just makes the GPU, it's up to many different manufacturers to actually put a card together. eVGA is a good quality card manufacturer. Beyond that likes already been said they have really good OC'ing ability on the 6800GT because they put on a heatsink meant for the Ultra on a card that's less powerful, so you're able to OC it up to and possibly past Ultra speeds relatively easily.
Other than that, the differences are just the quality of the manufacturer. Some are just superior products compared to the same cards of other brands.