The QX6660 will bottleneck your GPU's at 2.66 GHz running a "tri-crossfire" at high resolutions. You will need to overclock it to at least 3.4 GHz for better performance.
Your cards will run at 16x, 16x since it is two motherboard PCI-e slots. The 4870x2 is a one slot card, so it will run at 16x and the 4870 is a one slot card, so it will run at 16x. If you were running three separate 4870's, they would run at less speed, as it is using three PCI-e slots.