I have recently purchased a motherboard better but if you sli its only x8 if for both if i have twon card but one is is x16 but i can get a another mobo that has x16 x2 is better or does it even matter?
Note: have not built the pc yet.
The CPU can only handle a certain number of PCI-e lanes. The number is dependent on the CPU.
If it's a mainstream Intel processor (e.g. i5-4690K, i7-6700K, etc...), it can only handle 16 lanes. So even if you have two x16 slots wired for x16, the processor has to distribute the 16 available lanes to both cards, resulting in both running at x8.
This is why it's recommended to use a higher end CPU (e.g. i7-5820K, i7-5930K, etc...) when you're doing more than two way SLI or Crossfire, because you'd end up with cards running at x4 speeds, which is limiting. These higher end CPUs have more PCI-e lanes.
On the AMD side, the PCI-e lanes are handled by the chipset, so the motherboard does have an effect. For example, the 990FX chipset has 38 lanes and can handle 2 cards in x16, or 4 cards in x8, but the 990X chipset has 22 lanes, so it can only handle one card at x16, or 2 cards in x8.
If you're on the Intel platform, since Ivy Bridge, they've used PCI-e 3.0. It has double the bandwidth of 2.0, which is what the older AMD chipsets use. So, a card running at x16 on PCI-e 2.0 has the same bandwidth as a card running at x8 on PCI-e 3.0.
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Thank you
I believe it is the north bridge that determines the number of PCI-e lanes. On Intel chips, as you mention, the north bridge is incorporated in the CPU itself.