I read somewhere the asus MoBo does not have the x16 pipelines on PCI express like they say. So maybe check if you can find another MoBo that does support the X16.

googled this : http://enthusiast.hardocp.com/articl...50aHVzaWFzdA==

They dont seem to mind the x8 on the gfx port. The review seems highly happy with the MoBo.