I'm looking for a good motherboard to crossfire 2 HD 6970's and since i have no clue what would be good for crossfire it will probably be best to get some opinion's. When searching for motherboards i see that some are x16, x4 or x16, x8. Whats the difference? I have about a $150 dollar limit and a hight limit of about 2 1/2 inch near the cpu socket due to my cooling tower. So can someone explain the difference in performance between the different slots and point me in the direction of some good motherboards?