It depends on the game. If the game only uses 2GB of RAM while you're playing and you have 4GB of RAM installed, then adding more RAM isn't going to do anything for you. Check what your RAM usage is while you're gaming. If it's at 90% or higher, then you MIGHT see some (very small) improvements by adding more RAM.
If you're gaming hard and your RAM usage isn't maxing out, then adding more isn't going to help you at all.
-Wolf sends