12GB of RAM for gaming is overkill, but since you have a 1366 Intel chip you need tri-channel memory, so 4GB wont work for you. You need at least 6 GB (3x2GB), but you can run 12GB if you really want. Realistically, you are just throwing your money away.
Since you seem to be aiming for the best, you'll want low latencies (7-8) and high speed (1600 MHz).
DDR3 1600 is simply more available than DDR3 2000, and cheaper. Honestly the speeds will not make that much of a difference, and usually to get 2000 speeds requires mucking around in the BIOS (though not always). What I've heard at least, is that it can be more trouble than its worth, especially when you are paying a price premium.
Furthermore, if you get good 1600 memory, you can overclock it yourself to 2000 if you want to really see if it makes a difference. Basically 2000 memory is merely certified to be able to be overclocked to that speed, and as I said, often you have to do it yourself.