It's a bit more complicated than that. More VRAM per card generally let's you run higher resolutions. The rest depends on the GPU. I've always run with AA at 4x or 8x, never really since a difference above that and a 970 should handle 8x at most resolutions without a problem. With two you can increase the other graphics options.
Keep in mind that any effects depend on the games you run and the resolution you run them at. Without that information nobody can give a useful answer.