I have a i7 2700k, a XFX 7970 and the Asrock gen3 z68 Mobo. I eventually want to get a second 7970 and have a crossfire. From what I've seen running a 7970 at 8x pcie 3.0 will have almost no affect on performance, but i didn't take into account that the sandy bridge cpu only supports pcie 2.0 & 2.1. So, if i was to run crossfire would the second (or possibly both) 7970(s) be running at 8x pcie 2.0?
Yeah, both would run at PCI-E 2.0 X8, but you'll only lose about 3 frames per second. It's not really a bottleneck. I have a 2600K with two GTX 580s and they work fine.
thnx i just looked up a few charts to conferm this, and from what I've seen it will only reduce the performance by about 2%.
I used these sites:
http://forums.anandtech.com/showthread.php?t=2244211
http://www.trubritarforums.com/index.php/topic/8783-techpowerup-explores-pci-e-bandwidth-limitations/
http://www.behardware.com/art/imprimer/850/