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The lanes are fixed, but you have 2 x16 slots that support PCIe 2.0 now.
 
thanks for the clarification Eidorian
Tom's Hardware did some impressive tests on PCI-Express lane scaling. x4 lanes was enough in the majority of cases for gaming performance. You'd get more FPS for x8 or x16 lanes but it'd only be a few more.
 
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