Speaking for myself. I currently have 8gb of ram and I notice when my ram gets full the game will start to stutter more often. I don't think you will notice much of an improvement in the game unless you have lots of stuff running on your computer. I am not sure on a Windows but on a mac you can see how much ram is being used.
I try to keep the circle mostly green (which tells me how much free ram I have available) when I play.
I would try to find a similar app to tell you how much of your ram is being used. If you find that you are maxing out your ram then I would upgrade. If you are not maxing out your ram, then I don't believe you will notice much of an improvement.
I am no expert on the subject. Just my two cents.
Elk
Edit: You may benefit more by increasing the speed of your ram instead of increasing the capacity of the ram. Here is where I would start looking.