yes, you can do sli or crossfire. im pretty sure it has to be the same kind of slot tho. if you got two pci-e 2.0 cards. one in the x2.0 slot and another in the x1.0 slot, the x1.0 slot card will only run at the x1.0 speed, opposed to the x2.0 card slot.
ps. im only a novice when it comes to slots, but thats what i got from looking it up. hope this helps. ^_^