I would only suggest that first get a good idea about crossfire tech , before even thinking about utilising it .
I'll give you a start. Crossfire is the dual card technology invented by ATI , it enables the functioning of 2 ATI graphic cards on a single motherboard.
I would suggest just do a search on google and once you have a complicated question or any doubts then post it over here .
Your question is like coming to a car tuner and asking what is an engine :roll:
See what i am saying , learn the basics and then whatever is missing or you are confused about you can post it over here .