I see a few options here:
1. Adding another GTX 660 would be the best option IMO, if your MB and PSU allow it.
2. You could replace the GTX 660 with a $340 card, such as the GTX 770http://www.newegg.com/Product/Product.aspx?Item=N82E16814130946
The difference would be like 34 fps to 54 fps. Of course it depends on game, settings, resolution, CPU.http://www.bit-tech.net/hardware/graphics/2013/11/27/battlefield-4-performance-analysis/3
3. replace the GTX 660 with a GTX 760 or a GTX 660 Ti. That would be rather stupid, because the difference is too small to be worth it.
4. wait until GTX 780 cards become cheaper or you have more money.