I have a pc with a 8 gb memory and believe me its more than enough to play modern games.
For gaming 8GB of RAM is more than sufficient.
16GB is the usual amount for editing machines, though if it was a proper workstation rig, 32GB isn't that uncommon.
Though the CPU is a bigger decider in rendering performance than RAM is, pairing 32GB of RAM with a Core i3 will still be slow.
If you are just gaming, you wont come anywhere near using the whole 8. I've never been over 5 running a single game. I did read recently that getting more than 8 can help save your ssd from excessive page file writes and will imrove life of it some. Performance wise though, you will not need over 8.
4gb will provide enough ram to run current games like crysis, bf3 and etc: reason most people us the 4x2 (8GB) ram is if later they want to upgrade to 16GB (4x4) then it will allow them to have some better performance in multitasking. while if you run 4GB (2x2) or (1x1) you could only reach a maximum of 8GB for the (2x2) setup but can still get 16GB with the (1x1) module.
Easy answer is that with 4gb you are ok but if you did choose to upgrade you would see some speed improvements but not likely a huge fps difference in fps.
I have to disagree with the previous posters, if you monitor your RAM usage while gaming with 4GB is will report that you are not using all 4GB. This is because the OS is reserving some RAM for just in case situations. Now if you upgrade to 8GB or RAM and monitor the usage you will notice it actually uses more than 4GB of RAM. This is because now the OS has plenty to work with and can give the game all the resources it's asking for. I consistently use more than 4GB of RAM while gaming and in some game it can go up to around 6GB usage.
I recommend getting the extra 4GB, for the price of RAM it's always a good buy. You will not need more than 8 though, right now that is overkill for gaming purposes.