Bang for the buck upgrades are RAM and SSD as far as speed goes. I am running a 760 myself and am happy enough with it, but I am not playing cutting edge games, nor do I worry about running my games on highest settings. A 960, 970, or 980 would give games a very nice boost.
So... a minimum of 8GB of RAM, your games on a SSD (as well as your OS if you can swing it, either on the same SSD or a separate one), possibly a beefier GPU.