It varies by motherboard, and the specs need to say that the motherboard supports SLI and/or Crossfire. Just because it supports one, does not mean it supports both. All Intel X58 boards and most P55 motherboards support SLI, most will also do Crossfire.
You can get hacks that will enable sli on ATI chipsets and many Intel chipsets. If it can run crossfire and you got the bridge then you are set. When booting into os it will ask for normal windows or sli enabled windows the choice is obvious. It works and scales like normal sli. Most people don't know and their minds are like stone any way so I will try to find you the link. Only bother if you already have one card and Really want to try this.
Worked on my el cheapo 790x SB600 board with two 9800gt 1gb.