2nd GPU in x8 Slot

Can some one please help explain what this means in terms of running a second GPU in the x8 PCI-E slot.

"1 x PCI Express x16 slot, running at x8 (PCIEX8)
(The PCIEX16 and PCIEX8 slots conform to PCI Express 3.0 standard.)
* Whether PCI Express 3.0 is supported depends on CPU and graphics card compatibility.
* The PCIEX8 slot shares bandwidth with the PCIEX16 slot. When the PCIEX8 slot is populated, the PCIEX16 slot will operate at up to x8 mode."
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  1. Your motherboard has two pcie slots
    When one slot is used the cards will get x18 speeds when both the slots are used they will get x8 speeds.
    A 2.0 pcie card is compatible with pcie 3.0 slot and vice versa.
    When both pcie slots are populated the card may work as follows
    1st card at x16 and 2nd at x8
    Or both at x8. This depends on the motherboard. Some motherboards like ASUS rampage IV(the one which is over $400) runs all 4 slots at x16 whether they are in use or empty.
  2. Thanks. So what I undersand here is that currently in my Gigabyte motherboard with my single HD 7970 GPU it is running at x16

    If I where in the future to add a second HD 7970 they would now both run at x8. "When the PCIEX8 slot is populated, the PCIEX16 slot will operate at up to x8 mode"

    So slowing down my first GPU by half, how much gain would one get from the second?
  3. no you will not notice any slow down.
    pci-e 3.0 x8=pci-e 2.0 x16

    moreover the speeds that the slots can provide is way beyond the rate of data the cards can generate even when in x8 speeds
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