If you are not seeing the frame rates you want, try lowering the graphics settings to see if the graphics card is the bottle neck (higher FPS at lower settings) or if the CPU is the bottle neck (lower settings with the same frame rate).
At 1920x1080 resolution, the GTX 760 should be giving you close to the highest settings running smoothly.
If you do decide to upgrade, the GTX 780 should give you an improvement of about 45% over the GTX 760.
Personally, I would wait for the next generation of cards.
Crossfire gives unreliable frame rates. Avoid it.