Crossfire basically means that you can have two graphics cards of the same type in the same motherboard, therfore having twice the power (dual graphics card) im not entirely sure my self but i think that crossfire is ATI's name for a dual graphics card enabled motherboard and SLI ready is nVidia's name for the same thing.
Well thats my understanding, would anyone else like to dulve deeper?
its not worth it I have it in a few of my PC's.It about a 30% gain in performance and its mostly make games look smoother.If you got the money go for it but for twice as much money its not really worth it.
Theres a sticky around TF's about this called SLi the cold hard truth or something like It will tell you all you need to know.