As for your current setup, if modern games start to become too much for you, then you should probably play it on the safe side. I recommend getting a new card. The reason I say this is because new cards use newer technologies, and they have significantly more memory.
If you were to simply add another 7850 to your build, you'd end up disappointed in a few moths, when you see that the combined 2 GBs of GPU memory isn't sufficient for any new open-world games, and when you begin to experience jittery performance in games running DirectX 12 or Mantle.
A new single GPU, such as the beastly Radeon R9 290X, comes out of the box with breakneck clock rates, all the bells and whistles of new graphical shaders and technologies enabled, full future support for DirectX 12 and Mantle, and a whopping 4 GBs of GDDR5 GPU memory. All this without having to upgrade your power supply with an older CrossFire setup.
Hope I helped! If you need anything else, feel free to ask!