Hi I am building a new gaming PC,using a Amd 7970. It uses PCIe 3.0 x16. And all of the motherboards I am looking at, only support PCIe 2.0 x16 and x4. Will this be an issue? Should I try and find a cheap board with PCIe 3.0 x16?? And would my 7970 use up 2 PCIe slots or only one?? (Sorry, this is my first time building and learning PC parts) if not, what would be a scenario where I would use both PCIe slots??? And are there any other computer parts that use PCIe ports?? Thanks, sorry for the amount of questions...
They are backwards compatible. You'll be fine.
Oh Ok thanks, but just to clarify, it will be fully compatible, and there will be no performance hit?
With a single 7970, there will be no performance hit.
OK thanks
PCIe 3.0 devices will work in PCIe 2.0 slots, but with reduced bandwidth. In the case of video cards, there have been tests that show this doesn't matter, and there have been tests that show it does. For future proofing you would do well to find a board that supports 3.0.
The 7970 is a dual slot card, but that does not necessarily mean that it will take 2 pcie slots on your motherboard. A lot of boards are set up so that you can put a dual slot card in and not cover any other slots, leaving room for other cards to be used.
Other devices that use PCIe slots are things like sound cards, RAID controllers, and ethernet cards, though the vast majority of people only use them for graphics cards.
From all the tests I have seen GPUs even in crossfire/SLI do not need more than about 8x PCI-E 2.0 to meet their top performance in games. However those same tests also showed that compute programs were sometimes very limited by bandwidth and scaled up in performance all the way to 16x with PCI-E 3.0.
Its not really worth worrying about too much, a 16x PCI-E 2.0 slot isn't going to limit a GPU in a practical problematic way. Actually the question I presume is a proxy for a platform question, as in should I upgrade my CPU and motherboard or should I choose X79 instead of Z87.
So in the end the answer is yes it will and no it wont. Mostly the answer is no, for what most people do with a GPU PCI-E 2.0 is more than capable of letting the GPU perform as it should.