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Why do some boards have both pcie3 AND pcie2 slots? Do I need both?

Hi there, I'm just getting into building a new game system for myself.

I've been looking for a new motherboard which has PCI Express version 3 with 2-or-more x16 slots, and I have seen some boards that have slot(s) for PCIe3 AND slot(s) for PCIe2.

For example... http://www.newegg.ca/Product/Product.aspx?Item=N82E16813157477

I don't understand why, if PCIe3 is backwards compatible, there would be a slot dedicated to PCIe2.

Should I be looking for a board that has at least 2, PCIe3x16 slots, and if I happen to buy a card that is PCIe2 just plug it into the PCIe3 slot, or is getting a board with both PCIe3 and PCIe2 a smarter move?

Version Agony.
2 answers Last reply Best Answer
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  1. It's all backwards compatible. You won't see many PCIe 3.0 only cards. If you SLI two cards, sometimes some boards will drop both to PCIe speeds.

    Honestly; the PCIe slot is very fast. GPU's are not even close to pushing it's limit.

    A 16-lane slot (each direction):

    v1.x: 4 GB/s (40 GT/s)
    v2.x: 8 GB/s (80 GT/s)
    v3.0: 15.75 GB/s (128 GT/s)
    v4.0: 31.51 GB/s (256 GT/s)

    SATA6 (your harddrive) is only 500 MB/s. Ethernet Cat5e is 1Gb, which is like 125MB/s
  2. Best answer
    If your gaming will be on a single monitor, then a single X16 slot is sufficient to hold a very strong graphics card.

    Only the very fastest graphics cards are impacted by pcie 2.0. And that is on the order of 2%.
    Not a significant issue.

    Added slots might be useful if you are planning on dual cards.
    Then two running at X8/X8 will be ok.
    The example you linked showed the second slot as running at X4 which would not be optimal.

    Here is my canned rant on planning for dual cards:
    -----------------------------Start of rant----------------------------------------------------
    Dual graphics cards vs. a good single card.

    a) How good do you really need to be?
    A single GTX650/ti or 7770 can give you good performance at 1920 x 1200 in most games.

    A single GTX660 or 7850 will give you excellent performance at 1920 x 1200 in most games.
    Even 2560 x 1600 will be good with lowered detail.
    A single gtx690,7990, GTX780ti or R9-290X is about as good as it gets for a single card.

    Only if you are looking at triple monitor gaming, or a 4k monitor, might sli/cf will be needed.
    Even that is now changing with triple monitor support on top end cards and stronger single card solutions.

    b) The costs for a single card are lower.
    You require a less expensive motherboard; no need for sli/cf or multiple pci-e slots.
    Even a ITX motherboard will do.

    Your psu costs are less.
    A GTX660 needs a 430w psu, even a GTX780 only needs a 575w psu.
    When you add another card to the mix, plan on adding 200w to your psu requirements.

    Even the most power hungry GTX690 only needs 620w, or a 7990 needs 700w.

    Case cooling becomes more of an issue with dual cards.
    That means a more expensive case with more and stronger fans.
    You will also look at more noise.

    c) Dual gpu's do not always render their half of the display in sync, causing microstuttering. It is an annoying effect.
    The benefit of higher benchmark fps can be offset, particularly with lower tier cards.
    Read this: http://www.tomshardware.com/reviews/radeon-geforce-stutter-crossfire,2995.html

    d) dual gpu support is dependent on the driver. Not all games can benefit from dual cards.

    e) dual cards up front reduces your option to get another card for an upgrade. Not that I suggest you plan for that.
    It will often be the case that replacing your current card with a newer gen card will offer a better upgrade path.
    The Maxwell and amd 8000 or 9000 series are due next year.
    -------------------------------End of rant-----------------------------------------------------------
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