As you can see from the last two lines, the max bandwidth that the card is able to put across the PCI-E lanes is about 6.7 GB/s. According to the Wikipedia page that is a little more than 1 GB/s less than the max of PCI-E 2.1. These results were achieved with an i3-540 and a HD5870. Both are slightly overclocked but nothing crazy. I only hope that someone running a fancy i7-2600K with a pricey HD6990 can fill in that remaining bandwidth. If anyone has such a rig, please be kind enough to give PCI-E Speed Test a run and post your results.
I am curious about the typical size of objects passed to the GPU while gaming. The larger files post near that bandwidth limitation of PCI-E 2.1, but the smaller files are not even close. I would expect objects being passed in game would be many small objects which would lead to the slower performance like we see in the earlier stage of the test run. Perhaps someone with more knowledge of how objects in real world gaming are referenced could comment on the typical performance we would expect to see in game.
If my suspicions are correct, I wouldn't expect a new line of PCI-E 3.0 GPUs to be all that much of a game changer. Any comments and/or corrections to anything I've said are most certainly welcome as I am no expert in this area. These are only my observations and conclusions.