can someone please tell me what is performance difference between crossfire(two amd 6970) setup using PCI Express 2.0 (x16,x16) vs PCI Express 2.0 (x8,x8)? thank you for your help! and why only GIGABYTE (GA-P67A-UD7) Support this setup?
I looked up some reviews on this a couple month ago that you can probably find on Google. The general findings are that it will cost you 8% or so on your CrossfireX efficiency (184% vs. 192% if both are 16x). A x4 lane will cost you another 10% or so. I'm just estimating off of memory. But the overall effect is that it doesn't cost you enough to matter much with slower cards (like maybe a 5830 or GTX460).
Top of the line cards though? I can't tell you for certain. You can save money and get 6950's then flash the bios to 6970's. And get a better motherboard that supports two x16 lanes. If you're spending $740 on graphics, you want to get every drop out of it or you might as well be getting dual 5870's--which is what I'd do and save $an extra $250 that I'd spend on a 28" 1920x1200 monitor (although you probably already have a wicked monitor setup).
Baseline is 100% and we're comparing SLI scaling from 100% to 200%. So if it scales 92% worse, then it's still at 100%. If it scales 8% worse, it's at 184%. My math was right from that standpoint. But I guess the question was x16/x16 vs x8/x8 and I was answering x8/x8 & x16/x16 vs. single card. Although crossfire efficiency, which is what I was referencing, only goes from 100% to 200% (except for when it goes outside of that range) and so it was 8% less efficient as a crossfire setup.
So was my math right? Depends on what you think I was trying to say.