You don't need 16GB of RAM, 8GB is more than enough. Your GPU isn't half bad either, it won't max out most games, but it still runs games nowadays pretty well. I'm worried about your CPU/Motherboard.
If This is your exact RAM, just buy 4 more Gigs and you'll be fine.
If you have extra $$, get these two things.
It also depends on your budget. Just tell me how much $$ you have left over from your case, and I can whip up a good budget upgrade in a few seconds.