uh jeah hi, i'm a little confused, since i am planning a build with double gtx 970s and i'venoticed that most motherboards I found that one pci express lane is 16x and the second is 8x. And i have NO IDEA what that means, I know that 16x has more bandwidth, wich means faster data-speeds, than the 8x.
So if i have dual GPUs, one in 16x and the other in 8x, would they both run in 8x? and if so is it that noticeable? does that have so big of an impact?
Your kinda right, 16x has double the bandwidth than 8x. You have two types of ports though (or terms used with them) physical and electrical.
When you have two GPUs occupying PCI-e lanes, it depends on the board to how they operate. On higher end boards you may have multiple PCI-e (16x physical) connectors. With the first populated it will run at full 16x speed. If you add another (depending on chipset) the second may run also at 16x or 8x, whilst the first still operates at 16x. Or they might both drop to 8x.
PCI-e has an incredible amount of bandwidth in it, so you won't notice any performance degradation really unless you go below 8x PCI-e Gen.2 with most cards.
Like zanginator said, there's very little difference between PCIe 3.0 x16. PCIe 3.0 X8, PCIe 2.0 x16, and PCIe 2.0 x8 for modern graphics cards.
Puget Systems did it with Titans and it's within a 2 FPS difference on every test:
http://www.pugetsystems.com/labs/articles/Impact-of-PCI-E-Speed-on-Gaming-Performance-518/
http://www.techpowerup.com/reviews/NVIDIA/GTX_980_PCI-Express_Scaling/
Bugger all difference in most scenarios. I run my 980 in a 8x slot whilst having my 16x slot empty, purely for aesthetics. There's no difference from the benchmarks I ran with it in the 16x slot.