Crossfire means it supports two ATI/AMD video cards. The latest cards are the model 4850 and 4870. The 4850 is a very good deal and if you eventually get two of them it will be plenty fast.
Since that particular MB has onboard AMD video, it wouldnn’t be a good idea to use an Nvidia video card in it. But with a MB that supports crossfire, it can only run one Nvidia video card, but no one would really do that though.