You can get a 570 for about 250 these days that will max everything at 1080p or slightly above (like my 2048x1152). The more power you have the more FPS you have, which means you have a smoother gameplay. You can't just think about max FPS though, because you need to think of low dip. When a heavy scene hits your card it can dip to lower than 40FPS which could cause you lag. Having a beefier card will minimize the chance of that dip. Another thing for using more powerful cards is some games like Skyrim which you can totally mod out in HD textures can either max your VRAM or adding things like Anti-Aliasing can seriously tax your GPU. You also have even more image pretty toys to play with like Transparency AA, Ambient-Occlusion, Vsync, Tiple Buffer, Anisotropic-Filtering, ect that when all added up can bog even the most powerful cards in the most demanding games.
Doubling up on those cards even further reduces your chances of lag, and on any Nvidia card below the 600 series gives the ability for 2D and 3D surround. Triple monitor gaming greatly taxes GPU power and VRAM which is where SLI or Crossfire comes in handy for super high resolutions.
