You basically just buy a mobo that SLI (Nvidias version or Crossfire (ATIs version) Ready... the motherboard will have two PCIe 16x slots and you have to have two cards with the same Core (I.E two 7600GTs, but like a 7900GT and a 7900GTX wont work together), there is a special connector that you put on the cards. then you just enable it on your control panel and it increases your performance significantly.