SLI

From IexWiki

NVIDIA

In regards to NVIDIA's SLI -- Scalable Link Interface - This method use the PCI Express slots to connect two video cards together and reduce the total overall load on one GPU, theoritically improving performance upto 200%. The reality is, at the moment the performance increase (in terms of users who can't afford it easily) doesn't outweigh the hefty price to go SLI and a lot of games aren't fully capable of supporting SLI yet.

3DFX

In regards to 3DFX's SLI -- Scan Line Interleave - This method was for the Voodoo 2 chipset video cards. Two could be connected to go via PCI slots and one card would render every other line and the second card would render the one's the first card neglected to render, reducing overall load.