Theoretically, a single card with 2 DisplayPort output connectors could drive an 8KX. However, it would need to be a very capable GPU (like a hypothetical 2280). For the upcoming GPUs, a 2080Ti might be able to do it. I suspect that for now, you'd need 2 2080 GPUS at the minimum, although 2 1080Ti GPUs might be enough. It will depend on the game and the quality settings, as well as your tolerance for lower quality settings.