That's a good motherboard. Currently using it for my fan-cooled dual 6990 rig. I plan on adding more cards and will probably only end up hitting a wall due to my power supply and the amount of heat I can dissipate. You could run your cards in a PCI-E 1x slot and your Mh/s wouldn't be affected one way or the other. All the 1x/8x/16x denotes is amount of bandwidth available. Mining is computationally expensive, but very inexpensive in terms of bandwidth.