You don't get to decide what cards do what things. That's all handled by your CPU, the GPU drivers, and the game engine. If you've heard about using a dedicated card to handle effects, it's probably Physx, which is a proprietary Nvidia technology that's only available in a handful of games anyway.
A 5670 is probably like 1/8th as powerful as a 270x, so even if you could crossfire them (you can't), you wouldn't want to. But don't sweat it, a 270x should handle BF4 nicely.