Well, the results of my PCI Express 2.0 versus 3.0 on my 4-way SLI GTX 680 FW900 Surround setup are in. The results are so incredible I had to start the tests over from scratch and run them multiple times for confirmation!
3960X @ 5.0 GHz (temp slow speed)
Asus Rampage IV Extreme with PCI-E slots running 16x/8x/8x/8x
(4) EVGA GTX 680's running 1191MHz core, 3402 MHz Memory
nVidia Driver 301.10 with PCI-E 3.0 registry adjustment turned on and off for each applicable tes
After PCI-E settings changed, confirmed with GPU-Z:
All settings in nVidia control panel, in-game and in benchmark, EVGA precision are all UNTOUCHED between benchmark runs. The only setting adjusted is the PCI-E 2.0 to 3.0 and back and forth for confirmation (Reboots obviously for registry edit).
I kid you not, that is how much PCI-E 2.0 running at 16x/8x/8x/8x versus PCI-E 3.0 bottlenecks BF3 and Heaven 2.5 at these resolutions. I attribute this to the massive bandwidth being transferred over the PCI-E bus. We are talking 4-way SLI at up to 10-megapixels in alternate frame rendering. Entire frames at high FPS are being swapped and PCI-E 2.0 falls on it's face.
The interesting part was that while running PCI-E 2.0, the GPU utilization dropped way down as would be typically seen if you are "CPU" limited. In this instance I am not CPU limited, nor GPU limited. We are really at a point now that you can be PCI-E limited unless you go PCI-E 3.0 8x (16x PCI-E 2.0) or faster on all GPU
's in the system. GPU utilization dropped down into the ~50% range due to PCI-E 2.0 choking them to death. As soon as I enabled PCI-E 3.0, the GPU utilization skyrocketed to 95+% on all cores. I was going to run more benchmarks and games but the results are such blow-outs it seems pretty pointless to do any more. It may interest some of those out there running these new PCI-E 3.0 GPU's in which they think they are CPU limited (below 95% GPU utilization) yet might have PCI-E bandwidth issues.
Down to the nitty gritty; if you run a single GPU, yes; a single 16x
speed PCI-E 2.0 slot will be fine. When you start to run multiple GPU's and/or run these new cards at 8x speed, especially in Surround/Eyefinity, make sure to get PCI-E 3.0.