I'm also highly recommending the GTX 970!
Bought it for my PC that otherwise has about 5 years old components, CPU and all that. Yet almost every game runs with the maximum graphics. Most of the games are GPU driven after all. Arma 3 is literally one of the few games I am not able to run on maximum, because my CPU lacks the performance and I have only 8 GB of memory.
I got a bit more expensive version of the GTX 970 with TwinFrozr -cooling (since my previous GPU had this and it was noiseless and effective on what it did.) and if I remember correctly, it was some sort of a "GAMING 4G" edition. Had about 80 bucks more price than the regular editions.
GTX 980 is also viable option, but if you're not ready to throw that much money in it and if you don't mind overclocking your rig anyways, it is rather useless and does not meet the price tag.
Encore: You don't need to upgrade your whole rig! I'm still running on without an SSD, just 8 GB of some crappy old DDR3-memory, i5 CPU that wasn't even the "toughest" 5 years ago and so. Only thing I needed to upgrade alongside with the graphics card was the power supply. I had something like 600W and have now upgraded to 850W so the GTX 970 gets all the power it needs. Surely I took some extra capacity there as well, so don't think the 850W PSU is a requirement ;)