Is your RAM using all 8GB when you're playing Dying Light? If it's not maxing the 8GB, then adding more will likely do little to nothing.
Changing to better RAM (faster doesn't seem to be possible here if your mobo is limited to 1333Mhz), i.e. lower CAS latency to reduce the overall latency might improve your minimum frames, if the following 4 (four) conditions are all met:
- game is CPU demanding, multi-player, latency is equivalent or better (not CAS latency, true latency) & not cpu/gpu bottlenecked
So in the case of your current RAM, it is 1333CL9 at it's best profile, has a CAS latency in nanoseconds (ns) of 6.752 ns. 1333CL8 = 6.002ns (-11.11%), 1333CL7 = 5.251ns (-22.22%) So as you can see, only small gains in latency without any increase in bandwidth, which again, will only help you if your RAM is not maxed/full during game play in a CPU demanding game.
Putting all the theoretical aside, when you are playing Dying Light what have you measured (instead of just observed)?
Some suggested things:
FPS: min, max, avg
GPU: load, temps, volts
CPU: load, temps, volts
RAM: load
MSI afterburner will give you most of this.
Upgrading your RAM (more RAM) will only help if you see FPS drops when your RAM is at 100% or near 100% load). I love getting new shiny things, but I also love knowing I got the right shiny things.
If your game stutters despite all of the above being fine, it could be loading from your HDD and BJJ's suggestion for a SSD upgrade could fix that.
Gaming is all about min/maxing - identifying the chokepoints and eliminating them for the least amount of money.