"Can you use SLI on a CrossFire board or can you use CrossFire on a SLI board ?
Well , in general , the answer is NO. But it's said that if you hack the drivers , you can use SLI on a CrossFire board or CrossFire on a SLI board.
Caution:There is no guarantee that if you hack the drivers then you use SLI on a CrossFire Motherboard or CrossFire on a SLI board, so do it at your own risk!
However,with the release of i7/i5,Intel announced twe new chipsets which are X58 and P55 that allow users to use CrossFire and SLI on the same motherbard.(However X58/P55 boards support only CrossFire,some only support SLI and some support both,so if you wanted to buy a new motherboard,make sure you check this first."