Looking for Ram for Asus G72GX Motherboard

I would like to get Ram for gaming on my laptop. I always like the best and fastest on anything but im not up on the ram stuff. I play MMOs and would like some more ram for gaming. My system will allow 12gb of ram. Not sure if I will go that high but if I do will I see a big difference in gaming? I currently have 6gb I was looking to get at least 2gb more but I may want to go 4gb more not sure depends on the price of the ram you guys suggest.

Thank you in advance!!