From Wikipedia:
"PCI Express, officially abbreviated as PCI-E or PCIe, is a computer expansion card interface format. It was designed to replace PCI, PCI-X (interface card interface) and AGP (graphics card interface). PCIe is based around serial links called lanes. The PCIe 1.1 specification supports x1 (pronounced "by one"), x2, x4, x8, x16, and x32 lanes."
From Microsoft:
"Is an n-lane PCIe slot compatible with a p-lane PCIe graphics card, where p > n? Where n > p?
You cannot plug an x16 graphics card into an x8 slot. You can however, if you wish, plug an x8 card PCIe card into an x16 slot. A p-lane PCIe card will work at some speed in an n-lane PCIe slot, where n > p. This is not true if n < p."
And I can't find any PCIe x4 graphics cards so I dont know what they are talking about that being for dual graphics. plenty of x4 RAID controllers out there though. Fancy RAID? lol