Assuming you have no intentions of gaming on 2 monitors, and just 1, with the 2nd monitor being for more desktop space, there is virtually zero impact on performance adding a 2nd monitor. If you plan to game across 2 monitors, don't plan to keep doing that, as you'll find out having the bezels down the middle of your view is not worth the extra side to side view. The GTX 1080 will allow for higher FPS or settings, but the GTX 1070 can manage too. The question is up to what fits your budget.