Hi newcomer and welcome to the Tom's hardware forum.
1- For a Crossfire two ATI GPUs, you need a mobo with chipset AMD
2- For a SLI two nVidia GPUs, you need a mobo with chipset nVidia
3- You can do a Hybrid Crossfire or Hybrid SLI, that is one nVidia GPU and one ATI GPU in a chipset AMD or nVidia.
for the thrid option, its mostly for a mid-low-end gc with igp...