No it's highly unlikely that next gen of new Vid cards will come anywhere close saturating an X16 2.0 PCIe slot as they can barely saturate an X8 socket now. The PCIe 3.0 spec is a marketing ploy right now as it won't be needed for a long time.
The additional power from the 3.0 slot means you can probably run a GTX 580 without external power connectors. The amount of data being sent to the GPU is not going to increase drastically. The bandwidth is required internall in the GPU.