For the most part, the card manufacturer does not matter, at least in my experience. However, some manufacturers include custom coolers with their cards, or factory overclocks, or more RAM, or extended warranties. These are all things you have to research when buying a card. For my part, when I buy a new video card (and I've bought a lot of video cards), I almost always just buy the cheapest one, as long as it hasn't accumulated tons of negative reviews.
There are 12 EVGA GTX 680 cards listed on newegg (which is kind of ridiculous).
They can be divided into tow categories: 4GB cards, and 2GB cards (default). In each category there are cards that have been factory overclocked to different levels. The cards with the highest overclocks (most expensive) call themselves superclocked cards and have custom, high performance coolers.
It's factory overclocked, which is nice, because that means it's probably a better binned product. Since it's the same price there is no downside to buying it overclocked (except maybe a little more power draw, bat the card is designed to handle that).
Picking between the various manufacturer offerings can be difficult for the simple reason that it is intended to be. Vendors send out their best models to be reviewed and then produce similar models hoping ya won't know the difference.
1. First there's the basic "reference" design. it's the standard nVidia PCB, standard VRM, standard cooling system and usually no OC.
2. EVGA for example takes this card, adds a SC moniker and gives it a small OC. You pay $10 - 20 or so extra for them to apply the OC you could just as easily done yaself.
3. The next step up adds a better cooler and a little bigger OC.
4. Climbing another step the vendors add extra phases to their VRM to handle more and maintain more stable voltages. These can handle a bit more OC. The MSI Twin Frozr is a good example of this.
5. Another step up adds a custom PCB (likely some nice heatsinks) , even more phases on the VRM and even better coolers. Good examples of this would be the MSI Hawk and Lightning series, Asus CuII series and EVGA Classified.
6. Finally, take the preceding series and use selectively binned GPU's and ya have the final step.....good example of this is the Asus 670 TOP model