Other implementations of SLI - Wikipedia

Other Implementations

Two GPUs on one card

In February 2005, Gigabyte Technology released the GV-3D1-68GT, a single video card that uses NVIDIA's SLI technology to run two 6600-series GPUs. Due to technical issues with compatibility, at release the card was only supported by one of Gigabyte's own motherboards, with which it was bundled.
Around March 2006, ASUS released the N7800GT Dual. Similar to Gigabyte's design, but mounting two 7800GT GPUs on one video card. Again, this faced several issues, such as high price (it retailed for around $800, while two 7800GT's were cheaper total at the time), limited release, and limited compatibility. It would only be supported on the nForce4 chipset and only a few motherboards could actually utilize it. It was also one of the first video cards with the option to use an external power supply if needed [1].
In January 2006, NVIDIA released the 7900 GX2, their own attempt at a dual-GPU card. Effectively, this product is a pair of slightly lower clocked 7900GTX cards bolted together into one discrete unit, with separate frame buffers for both GPUs (512MB of GDDR3 each). The GeForce 7900 GX2 is only available to OEM companies for inclusion in quad-GPU systems, and it cannot be bought in the consumer market. The Dell XPS, announced at the 2006 Consumer Electronics Show, used two 7900 GX2's to build a quad-GPU system. Later, Alienware acquired the technology in March.
A newer version, the GeForce 7950 GX2, addresses many issues in the 7900 GX2, and is available to consumers for separate purchase. More recently, in August 2006, NVIDIA has released the drivers to allow end-users to build their own quad-GPU systems using the 7950 GX2[1].

Quad SLI

In October 2005, Gigabyte Technology released the GA-8N SLI Quad Royal. Essentially it was a motherboard with four PCI-Express x16 slots. At the time of release however, NVIDIA stated that it would not be the direction it would take SLI [2].
In early 2006, NVIDIA revealed its plans for Quad SLI. Instead of combining two GPUs onto one board, two separate boards would be stacked on top of each other into one discrete unit, using the existing on-die SLI bridge connectors to send frame data. This way, four GPUs can contribute to performance.
The official implementations of Quad SLI work in the same fashion. Two GPUs are placed on two separate boards, with their own power circuitry and memory. Both boards have slim coolers, cooling the GPU and memory. The 'primary' GPU can be considered to be the one on the rear board, or 'top' board (being on top when in a standard ATX system). The primary board has a physical PCIe x16 connector, and the other has a round gap in it to provide cooling for the primary HSF. Both boards are connected to each other by two physical links; one for 16 PCI-Express lanes, and one for the 400MHz SLI bridge. An onboard PCI-Express bridge chip, with 48 lanes in total, acts as the MCP does in SLI motherboards, connecting to both GPUs and the physical PCI-Express slot, removing the need for the motherboard to support SLI. One or both boards features an SLI connector, depending on the model.
Quad SLI has yet to show any massive improvements in gaming using the common resolutions of 1280x1024 and 1600x1200, but has showed improvements by enabling 32x anti-aliasing in SLI-AA mode, and support for 2560x1600 resolutions at smooth framerates (Typically 50FPS or better) on maximum settings.
NVIDIA has recently released official Quad SLI drivers, marking the first time one can use Quad SLI with official support.[3] For more information, visit NVIDIA's Quad SLI website.

Physics Calculation

In response to ATI offering a discrete physics calculation solution in a tri-GPU system, NVIDIA announced a partnership with physics middleware company Havok to incorporate a similar system using a similar approach. Although this would eventually become the Quantum Effects technology, many motherboard companies began producing boards with three PCI-Express x16 slots in anticipation of this implementation being used.