the most you'll ever see in current AAA games. you might be thinking "well the Beta's using 9GB+ of memory".
it's a game in Beta; they're usually not fully optimized. so if you're planning to play a whole lotta games still in Beta, maybe, just maybe that 16GB may be worth it. but you really should be waiting for the final product for a better experience. and it sure saves you a decent bit of cash going with 8GB and waiting for a full game to come out.
This was the first link on Google for this question, so I'm gonna go zombie here.
After much reading, and watching my own systems ram usage (currently have 8GB G.Skill), I can say that 12GB-16GB is definitely the way to go if money isn't an issue. I for one don't want to close every program that I have running to play a game (typically using 4-5GB at all times), that's just annoying. The upgrade that I'm working on now will definitely boast 12GB, if not 16GB.
MediaMonkey, Steam, Origin (like I said, I don't like closing things), Cyberfox with 10-20 tabs open (that's 1.2GB alone) , Explorer, Pandora, Everything, Skype, MS Word (for notes, etc.), Lightroom (nothing like Photoshop in terms of resources, this is typically around 180MB), plus all those background goodies, like AV (Comodo), etc.. I'm currently at 4.2GB of usage. If I play something like Dead Rising 3 or Skyrim modded-out without closing anything/most everything, I'm hitting 85-98% usage.
And what I'm getting at is that I think it's a pain to have to close programs to play a game, shouldn't have to close everything on a PC... it doesn't make sense to do that imo.