On the top of the card there is a notch where the SLi bridge goes. Most of the SLi compatible motherboards come with an SLi bridge included.

You can get 2 Nvidia GeForce 7950GTX2 cards, which have a Gigabyte of memory and 2 chips on the one card. Having two of these in SLi creates Quad Sli, and requires a compatible motherboard, power supply, a big enough case (the cards are long) and another SLi bridge.

Alternatively, you could buy a single 8800GTX card, for about 3/4 of the price of the Quad SLi. A single 800GTX card beats a Quad Sli setup.