Since the vast majority of videogames are still 32bit, that means that the most they can allocate is 4gb in 64bit windows (3gb in 32bit), AND that is only if they support large address functionality, otherwise it's just 2gb.
On the other side, RAM is cheap, and more is better.
I'm currently sitting looking at my task manager and I can see I'm using 4GB with just Firefox 10 (42 tabs open and 22 addons) and an RDP session open. Now, all my other apps are what is pushing that RAM usage up. I have Ubuntu One, Dropbox, Logmein, Kaspersky AV, TrueCrypt, Origin, HTC Sync, Windows Live Mesh and Steam all sat in my control panel.
Having a look at task manager shows:
Physical Memory (MB)
These are dropping as I've just been playing BF3. I'm not even running any VMs, which quickly eats away at the GBs, however, I've never managed to see anything less than 4GB of RAM free. The reason that I added a further 12GB to my original 6GB was that I could see that in time I'd likely need it before I need a new CPU and 6GB was too much of a constraint in Civ 5 at the time.
6GB of ram is an overkill for most apps. Unless you do heavy video encoding or some scientific simulations or you like to work with 200 RAW images in PS at the same time there is really no need for more than 4GB.
The quality of ram ie clocks and tight latency is more imprtant than having say 16GB or ram that can't even do 1600Mhz at CAS7.