I've seen benchmarks for modern games like Arkham Origins that actually showed worse performance over 8 GB of ram, and the exact same between 4 and 8 GB.
Just stick with 8 GB regardless if its a gaming rig. After all, the PS4 and the Bone only have 8 GB. That will be the standard for the next 10 years.
Also lol, when CoD: Ghosts was released it wouldn't even install if you had less than 6 GB of ram, for no apparent reason. Worst optimized game I ever played.
Also, I suggest you upgrade to Windows 8.1 64-bit. It is much better optimized for modern games. Just pirate it if you have to.