16GB is optimal for gaming and other high workloads. If you're at it you should just upgrade to 8GB if you still have two or more of your slots free. Windows is horrible on RAM as a whole so that would actually help quite a lot.
Windows has actually gotten quite good at managing RAM. If you give it more RAM, it will increase its RAM usage to an extent because it uses some it as cache to speed up app loading So, putting more RAM in and seeing Windows eat up more doesn't mean its a memory hog, it just means it is finding a use for the new free memory. Other OS's do this too. Now certain apps made for Windows are NOT good on RAM (you know, that one program that eats up memory like mad until it is closed or crashes... memory leaks are a pain).
