A Ram question.

Supports four unbuffered DIMM of 1.5 Volt DDR3 800/1066/1333/1600/1866/2133* (OC) DRAM, 32GB Max

You should be able to upgrade to 8GB with no problem. I would suggest getting a 2 x 4GB kit. Lot's of times 2 different kits of the exact same memory will not work together. And manufacturers will not uphold a warranty for that either.

Each kit is tested to make sure the sticks will work together.
 
Well this gen's games will run if u have 8gb of ram, u can have 32gb of ram or 64gb of ram or 1tb of ram and it wont influence u much, if the game only needs 8gb of ram then it will only use 8gb of ram, so going from 4 to 8 might get u an increase but going any further at the moment wont
 
Put 1866MHz memory with tight timings (the lower the faster) and there should be some improvement. More memory might help some with stability while gaming too. The best and surest way to improve performance especially with gaming would be to upgrade the GPU.