Can't think of a game where you'll need 16GB for it. In most cases, even 8GB is plenty. As another fellow has pointed out, your $$ is better spent on a graphics card if you are trying to maximize your gaming performance.
Assuming that you are running on Vista/Win7, you can easily monitor how much RAM you are using with
desktop gadgets. Unless you start running out of real physical RAM and notice the PC swapping stuff to/from the HDD when you switch between programs, there is no benefit in having more RAM sitting around doing nothing.
When I am not playing games on the PC, most of the time I am only using around 2GB's worth of RAM.