It depends on the game, most games honestly don't take you much if any over 6gb, and unless you're doing a lot of things in the back the difference won't be dramatic. It will help but it won't be huge. Additionally if your other 3 sticks are all a mixed bag, adding another random stick to the mix might actually cause more issues than it helps. You are welcome to try and we can probably get you stable by changing voltages if it doesn't just work, but if I were going to 8gb I would buy a 2x4gb kit and drop it in, not piece together a 4x2gb kit.
heck you can get 8gb of pretty fast memory for less than 60 bucks right now on newegg.