Question about Sapphire Radeon RX 580 Pulse

I currently have this graphics card in my PC with 8gb Ram and an i7 6700. I use 3 monitors. Most of the time I use my main monitor for games, etc. But rarely do I use all 3 monitors for games. I have an average of 30-70 fps but now I want to achieve 120 fps in most games. Should I install a 2nd RX580 or buy another graphics card and use both or what would be best.