Hi catthing,
While it is good practice to get your RAM to work at the highest mHz, it is also not as important as some sites and memory manufacturers
want you to believe.
This article demonstrates how dual channel RAM did almost nothing in terms of real life performance (http://www.gamersnexus.net/guides/1349-ram-how-dual-channel-works-vs-single-channel/Page-3).
The Megahertz of your RAM is important but it wont give you more than 2-5% improvement in real life scenarios, but more important generally is how much RAM you are running in the system.
http://www.overclockers.com/forums/showthread.php/691850-1333-vs-1600mhz
I would stick with 12GB because windows does a good job at caching files and if you "sleep" your computer (Suspend to RAM STR) instead of switching it off it will cache more and more files until you hardly have to load anything.
I have seen people buy 32Gb of RAM and make a large RAM drive and copy their favorite game on windows load onto the RAM drive so they can get almost instantaneous load times. This is much overrated, as it is quite a bit of money and time involved setting it up, and the advantage vs a fast SSD is marginal, especially when windows already cached those files once. RAM drives can be quite effective if you are developing software for example, when you keep your files on the drive. But care must be taken to synchronize those files regularly to a real drive as if there is a power failure you will lose everything on the RAM drive.
Gabor