As you know, you can keep track of overall RAM usage via Task Manager.
In particular, Windows itself, may over the years, function a bit better with more RAM.
But... as mentioned above, if you have 16GB you should never be able to even get near to using that.
Various programs can seriously use RAM. Many browsers with many tabs open, image/video editing software... etc.
The secret to keeping Windows speedy, is to "keep it clean". For example...
Let's say you're testing a bunch of DVD ripping programs one day. After the testing, you can attempt to try to clean all the leftovers... OR, you can just restore from a backup, and then only install the program you've chosen.
Anything you do, in Windows or with 3rd party programs, leaves "tracks" all over the place. After a few years of this... Windows will start to slow down because of all the crud it has to deal with whenever you try to do something.
Intelligent use of backup software, can do wonders for Windows, and as we all know, for many problems that might arise.
But mainly.... backup software is very useful for keeping Windows clean and sharp. :)
/edit
Short version...
As soon as you first install Windows... make a full OS image backup.
Then test, play, abuse Windows all you want. When you're done and have made your choices of settings, 3rd party software etc... restore from a backup and use just those settings and 3rd party software, that you decided on.
Then make a fresh backup to use for the future testing, playing, abusing of Windows.
The "restoring" cleans out all the crud from the testing, experimenting, etc.
