I still think it's quite subjective and depending on user how much memory they decide to go with.
For clarity: I'm running one 8GB kit and one 4GB kit in dual channel on a P67 mainboard with a 2500K Sandy Bridge processor. That's how I got 12GB. I previously had a system with 4GB of RAM, and I can definitely tell the difference in multitasking without the aid of a stopwatch.
Loading levels once the game starts will also be positively affected, leading to a smoother-feeling game experience. The games themselves won't play differently but they will look differently with higher detail levels and less graphical anomalies like missing textures and such. This also makes for a better gaming experience. Any time a game has to pause/stop and load anything at all, more RAM makes it faster. These are not measurable with benchmarks and stopwatches -- they are subjective (but very real) differences.
If you get two kits of the same kind of RAM at the same time, you minimize (to as close to zero as you can get) the likelihood of incompatibilities and instabilities that can cause slowdowns. Of course, sometimes boards can't populate all four RAM slots unless you loosen the timings, but that tends to happen mainly to older boards.
I still recommend 12GB or 16GB.