SLI is available only with PCI Express cards and you need to have a motherboard with two (or three, in the case of 3-way SLI) x16 PCI Express slots and the motherboard must be based on an NVIDIA chipset (the only exception is the Intel Dual Socket Extreme platform – codename SkullTrail – which is based on the Intel D5400XS motherboard which supports both SLI and CrossFireX technologies). Keep in mind that depending on the chipset the x16 PCI Express slots can run at x8 speed when SLI mode is enabled (more on this later).
The communication between the video cards is done through a private connector, called SLI bridge. So usually the cards do not use the PCI Express bus to transfer data between them – and that is why using the PCI Express slot under x8 mode isn’t so problematic. The exception goes to mainstream video cards from the GeForce 6600 (except 6600 GT), 7100 and 7300 series, which support SLI but using the PCI Express bus to make the communication between the cards – so they don’t need an SLI bridge.
Originally to use SLI the video cards had to be identical – same GPU, same manufacturer and even same BIOS version. From GeForce driver release 80 on NVIDIA relaxed a little bit and now the video cards can be from different manufacturers, but they must be based on the same GPU. This rule is valid for all versions of SLI (SLI, Quad SLI and Three-way SLI).
All video cards from GeForce 6600 on support SLI, but three-way SLI is only supported by GeForce 8800 GTX, GeForce 8800 Ultra, GeForce GTX 260 and GeForce GTX 280. This mode uses a new bridge as the cards have two SLI connectors.
As for quad SLI it doesn’t use four video cards, but two GeForce 7950GX2 or two GeForce 9800 GX2 video cards installed in parallel. Since each video card has two GPUs each, your system will have a total of four GPUs.
The only “problem” with SLI is that when this mode is enabled only one video output is enabled, so you can’t have a multiple monitor configuration under SLI, just a single display.
Also even though in theory SLI was made to double the gaming performance in practical terms this doesn’t happen. SLI improves the performance of some games better than others, but not even close to 100% increase.