I run 12GB of DDR3-1600 CL9 in my Sandy Bridge overclocking/gaming machine.
I recommend 4GB as an absolute minimum, but 6-8GB for gamers who like better performance at higher resolution and graphic detail settings. The extra really does make a difference, despite what others have said. "
3) 4GB is optimal for most; ALL VIDEO GAMES have sufficient RAM available and will not benefit from more RAM during game play" is a blanket statement that is untrue with modern games, and will become less and less true with future games.
Read this:
Linky
The important part is below the pic of the ATi video card.
My GTX 570 graphics card has 1280MB of its own, but it also reserves 2815MB of system RAM to use for texture storage and such. In a system with only 4GB, it wouldn't have that extra storage space to count on, so performance isn't as smooth. The more system RAM you have, the more the graphics card can reserve, up to a point. Right now, 8GB is the optimal amount of RAM.
To see how much RAM your graphics card is reserving, go to the Windows Experience Index page and click the "View and print detailed performance and system information" link. It's listed under Shared System Memory.