please check with your MoBo manual as I could be wrong, but from memory PCI 1 and 3 are x16 slots, 2 and 4 are x8 and x4...........not enough band width for high end card me thinks!..........x8 with the 2x16 slots should work with the crossfire bridging bar but x4 slot might not have enough bandwidth........check my sums but I think this correct.
Aussie Allan