A RAM upgrade to 16GB will only realistically help in applications that require more than 8GB. If the games you are using don't require more than that, then the difference might be negligible (usually).
However depending on how your RAM is set up, moving to dual channel over single channel always yields a bit of improvement.
What resolution and settings are you playing on? The 2600 is good, but it is beginning to show its age now.
What make and model PSU is it?
How full is your SSD? Have you Run Virus scans? SSD trims? etc.
Are all drivers up to date?
have you monitored temperatures of individual CPU cores ?
Have you monitored GPU temperature?
Currently I am in dual channel, sorry for not mentioning
1080p, 144hz when its realistic
The PSU is a Rosewill HIVE Bronze rated
SSD is usually around half full, I have Bitdefender and scan regularly
Drivers for graphics card are updated regularly
GPU and CPU both hover around 50-55 C, never really looked at core temp, will do now though
There are some games that will happily use 8GB of RAM, so that is potentially a problem. That being said, as a former 2600k owner, the 2600k is getting very long in the tooth and is the most likely cause of your random slowdowns.