What is the motherboard that you are getting (you forgot to include the link above)? It must be have at least two PCie 2.0 x16 slots able to operate at X8/x8 mode in order to make the crossfire mode worthwhile.
Also, there is no reason to get a 6870 because the slower card will always limit the speed of the faster one. This is because Crossfire operates by assigning every other frame to one card and the rest to the other card. Therefore, even if the faster card renders a frame twice as fast, it can only process every other one so it effectively operates at the same speed as the slower card.