Right now, 8GB is enough. We're just now seeing games that actually use more than 4. I would think that any PC today would be fine with 8GB. If you need more in a few years, memory is probably the easiest upgrade to make anyway.
Right now its MUCH better to get 8 GB(4GBx2) in your system because games hardly ever use more than 6GB. And if you one day need more its very easy to get more.
Getting 16GB right now is almost throwing away money in my opinion. SO unless youre doing some other heavy tasks such as photo/video editing do NOT waste money on anything but a nice 4GBx2 set of RAM