ExtremeTech has posted an article on AGP 8X
Based on our initial testing, AGP 8X on today's architectures and with current applications doesn't appear to deliver any appreciable performance improvement for nVidia boards. The Radeon 9700's potential benefit from AGP 8X is still something of a question mark.Read more
This story may change down the road, it will take some time for game developers to architect game engines that use AGP 8X to produce significant performance improvements.