Crossfire and SLI is not compatible.
You can, however, install a dual GPU video card in either type of motherboard since effectively it is still a single video card. For example, you can install the $600 HD 5970 into your motherboard. It is basically a video card with two slightly downclocked (I believe??) HD 5850 GPUs, but your case must be long enough to fit the card (11" maybe longer).