I have a GTX 670 plugged into the PCIe x16_1 port as recommended.
Will plugging a Xonar STX PCIe x1 card into the PCIe x16_2 port of this mobo cause any slowing in the system? I am not sure if all the lanes are going to the first x16 slot or not... and I don't know if it knocks down the GPU to an 8x PCIe 2.0 level when you use a non-graphics card in its twin slot. The manual also says IRQ assignments are shared between PCIe x16_1 and PCIe x16_3, so I don't know if I should plug the card into the third x16 either.
The first and second PCIe x16 ports are BOTH version 3.0 capable.
(I'd rather not use the PCIe x1 ports because the card might receive extra EM interference from the graphics card).