TrustedReviews tested ATI's CrossFire with dual X850XT cards.
One of the most obvious physical differences between SLi and CrossFire, is the method of connection between the two cards. With SLi a small bridge is employed between the two cards that links them internally, while CrossFire uses a daisy chained DVI cable that links up the cards externally. There's no doubt that the nVidia solution is more elegant from a PC building point of view, but looks aren't everything. Read on over here.