Depends on what you can afford. 8GB seems to be the sweet spot, but theres never anything wrong with having more. I currently have 2 x 8GB sticks. Do I need them? Probably not. Is it nice having them? Damn skippy.
I had 8 GB for a while and recently upgraded to 16 GB and for gaming I cant tell a difference, however load times might be slightly faster. I would say get 8 GB if money is an issue and put the rest of the money into a better video card.
16GB is Hugely alot but depends on your working situation..if your going to play intensive games like GTA IV then 8 or even 16 would be good but if your going with BF3 and skyrim and those games..then 8GB will be enough