Windows won't use more than 2GB of memory.
32-bit applications (including games), even running on a 64-bit operating system can only address 2GB memory.
There is some reserved memory addresses as well, so you can use up to 6 GB when running most games.
Some newer games are coming out with 64-bit executables.
These can take advantage of your extra RAM, but I don't think any of these require more than 8GB at the moment.
8GB is enough for now, but 16GB might be a good idea to future proof your PC.
Despite earlier posts, trying to mix RAM modules can be problematic.
In a years time you may not be able to get the same RAM modules anymore.
This would mean to upgrade to 16GB later would probably mean removing the 8GB you buy now and...