I believe a hidden question within your question is :"Will I be able to enable Ubersampling on the Witcher 2?". On my GTX 780 TI, true max settings, including Ubersampling, runs around 40-55 FPS. So a single GTX 780 Ti is the minimum necessary for the Witcher 2.
The Witcher 3 is a GPU accelerated PhysX game, so an Nvidia card will be mandatory for maximum settings. Beyond that, we really need to wait and see how much it will push current hardware. I would assume you might need dual GPUs or the latest Maxwell card.