you don't need to upgrade if all you want to do is play a video game on one monitor and play a movie on another monitor. Movie files are CPU bound, not GPU bound. GPU is used for textures, floating point operations, ray tracing, etc. CPU is used for decompressing video content. I have a 680 and run games at 60 fps while running a movie fullscreen and having another monitor for web browsing. Especially for stuff like Diablo 3, Battlefield, etc. Yeah, GTA5/Witcher 3/Battlefront 3/Star Citizen/etc will require better GPUs to get max visual quality at 1080p 60 fps (or higher) but you will run them all at 30 fps and still be able to play a 1080p movie on another screen, no problem.
What you really want to upgrade for is when you want to do 4k gaming, or dual(+) screen gaming - ie mash two monitors together and play on both of them simultaneously like something out of the matrix or extreme flight simming.
i'd really, really just wait to upgrade unless your GPU dies and you need to replace asap. Reasons are (1) windows 10, (2) directX 12, (3) Valve/Oculus virtual reality. All 3 will drive changes to GPUs in the next few months, realistically we will see shitty v1.0 iterations of their tech this fall and better v2.0 iterations early next year. That'll be the sweet spot to buy. Yes, cards you buy now may have DX12 drivers but you bet your ass they won't be optimized as well as stuff that comes out later.