If you're running out of memory and programs are crashing, yes you should upgrade. If not, then don't worry about it. So games can take advantage of the extra ram. You'll know when you really need to upgrade.
It is hard to tell what you are actually using/needing. Windows keeps code in ram in anticipation of reuse.
If you have any significant demand page faults, then you have too little ram.
All games I know of are built around 32 bits which can use 2-3gb by itself.
Unless you are doing other multitasking, then 8gb seems to be the sweet spot.
That said, ram is cheap, I like 16gb. It seems to make a pc more responsive.