8GB if you have future plans/current plans of gaming. Your gonna want atleast 1600Mhz DDR3 RAM. Anything over 8GB's isn't really needed, once your at 16GB of ram you will never use it and going further would do you know good. Any ram over 1866Mhz also will not benefit you for the cost to benefit ratio and wont be a noticeable improvement.

G.Skill ares is a very good choice for ram.

Also if you have an intel CPU you wanna make sure the RAM you get will run at 1.5V anything that runs higher will void your CPU warranty and will destroy your CPU over time. Anytthing less than 1.5V is also fine. It's recommended you get 1.35 or 1.5V RAM.

6 GB runs it easy but im upgrading my laptop to 16GB soon

16 in my opinion is great for me but 8 is all you need for your gaming

Get 8GB, there's no reason to get 4GB as RAM is dirt cheap at the moment. You should try to buy higher frequency RAM (1800Mhz+ etc.) as you'll see a greater performance increase

Ziayn wrote I have 6gb and it runs Cod4 without a problem

lol cod is almost as easy to run as minecraft