If you have the GPU "ASUS GeForce® GTX 660 DirectCU OC: 1085MHz overclocked, 20% cooler, 3X quieter" then there shouldn't be any issue with your FPS given that you have all other components like RAM,Drivers etc upto date and functioning properly.
you have to check that what OS you use? 32 or 64 bit? win 7? As you know that 32 bit doesnot utilize more than 4GB Ram so it doesnot extended to max utilization of your RAM(16GB as you said).
You need to do provide more details on your RIG with Details on your GPU,MOBO,RAM,HDD. these are only possible factors to look closely.
YOu can try out
> Update drivers on Asus GPU from its panel, Optimized setting for games from the panel option available.check game.
> Go to Game setting. Bring to default settings of the game for play. Mostly that brings your any unwanted/Not correct setting changes you might have done to not really optimized settings in the game. then play. then try taking each level of low-Medium-High-Ultra settings in the GAME options and see what level you can play game with moderate game fps and a smooth transit of movements. Thats your optimized level of rig FOR the particular game.
> Check window index and find which component is lowest ranked.That can give you clue.
> if your system OS is performing for normal task. you might need to little tweak it and clear memories,fragments etc.
>Check if you have any error logs while booting or performing any task. resolve it.
> Check what all applications running in background like MSoffice etc, close all. you dont need them unwanted running in your background.
> Upgrading to 64 bit(Last option)
If you are still up with Issues, please drop more details on the Rig specs.there might be minimal hardware config issue with your OS/Drivers. etc