I have 16gb, it wasn't even intended lul.
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I asked this on another forum. Verdict is that it doesn't help games but can speed things up if you quickly tab out of the games while they're still running.
I had 4GB and was using 3.95Gb during BF3. Now have 8GB and can use up to 5.99Gb (I've seen) during BF3.
As BF3 itself is 32-bit (according to what I was told) it can only use 2GB itself anyway. It may help for running other programs alongside the game, but won't help the game.
And speed... utterly pointless with a fast CPU such as your 2700K!