You never want to game on an even number of screens (except for games like some RTSes that are programmed for it). Why? The center of your view will have the bezels from your monitors. Imagine playing a shooter where the most important stuff is obscured by a huge black line in the middle. Yeah, not fun at all. A 670 will handle 2 monitors just like it will handle 3 monitors (although you may have to lower some settings in some games), but you'd still only want to play your game on one, in which case there will be no noticeable performance difference. A second monitor is still useful for your mumble window, monitoring software, web browser, etc. but you won't want to run run games on two screens -- either 1 or 3.