PCIe Bandwidth & SLi... I don't understand it, help?

[DanJames93]

I'm in the process of upgrading my system. I am going from an AM3 Athlon II to a 1055 i7, and a GTS450 to a GTX570/80 or 670/80, possibly in SLi but I need some advice on PCIe bandwidths as I don't really understand them..

I'm looking at motherboards for around £120 that support SLi, but here's what's confusing me...

"Supports 1x PCIe 3.0 at 16x or 2x PCIe 3.0 at 8x"...

Won't one card running at 16x be the same as two running at 8x, as the motherboard is allowing half the bandwidth.

Or... would two at 8x be faster, because something tells me that PCIe 3.0 can support a much, much higher bandwidth than most (maybe all) modern GPU's can possibly support?

(I'm considering the ASrock Z77 Extreme4 by the way)... If anyone has suggestions, please keep it reasonable. I don't have stupid amounts of money to spend.

Thank you!

 

[DanJames93]

Imagine it as a speed limit.
16X PCI-E 2.0 won't get saturated by any GPU now.
PCI-E 3.0 basically has twice as much bandwidth, but it's like saying "We can now go to 120MPH, but our cars only go 30MPH", and then the PCI-E 2.0 16X would be 60 MPH, PCI-E 2.0 8X would be 30 MPH etc.

Thanks for the replies..

So, lets say a GPU will only use 25% of the supported bandwidth of a PCIe 3.0 slot running at 16x, it would theoretically take a total of four GPUs running at 4x before the full bandwidth of the GPUs (and motherboard's PCI slots) are used to the full extent.

I get it now.