Welcome Newcomer!
In most instances 1600 MHz (12800) is plenty good enough for gaming. The "value" of RAM is High Speed + Low CAS latency. The difference between 1600 -> 1800 MHz DDR is added cost and 2-4 FPS.
The minimum amount of RAM recommended for x64 Windows is 4GB (2x2GB) or for X58/1366 MOBO's 6GB (3x2GB).
Tested is memory the RAM Mfg has confirmed that worked on the particular MOBO in ADDITION to Certified RAM that the MOBO Mfg has confirmed & certified that works {XMP} listed RAM:
1. Certified RAM: MOBO Mfg {e.g. ASUS, GA, MSI, etc} tested and certified; including BIOS XMP recognized.
2. Tested RAM: RAM Mfg {e.g. Corsair,G.SKILL, Kingston, etc} tested and confirmed to work properly with a particular MOBO.
3. Pot Luck RAM: Seeing is believing, it is nearly impossible to know what RAM will or won't work. Further, most people cannot determine which RAM to choose correctly 9/10 times. So to "pick any RAM" is Poor Advice.