I right now have 1 GTX 580 in a system with a i7 2600k. I understand that the Sandy Bridge CPU's run video cards at 8x if 2 PCI-E video cards are installed. I was considering adding a 2nd GTX 580 in my system. I read a thread that PCI-E bandwidth is important for autogen in FSX. Is PCI-E 8x going to be much slower than PCI-E 16x in FSX if I have my autogen at the extremely dense setting?

You need to review your motherboard manual since it is a function of the motherboard. My R3E will run two at x16 and out of the four slots they have to be in the correct slots for both of them to run at x16.