sli (nvidia) crossfire (ati) allows you to put two video cards together onto one sli-able/crossfire motherboard. people do it cause they can buy one video card with the money they have now and when it comes time to upgrade, instead of tossing the card now and buyying a better one, they can just buy another of the same card or a better one (currently only with crossfire, but the better card would be downclocked so it matches the other card), put them together, and get a major improvment.