As far as performance is concerned 2 Dimms is all you need so that you can run dual-channel mode.
Extensive testing of both AMD and Intel CPU powered desktops has shown no tangible benefit to RAM frequency higher than 1600 MHz. when running REAL applications. Benches show "potential" benefits to higher frequency or lower latency RAM but because DDR3 RAM running at 1600 MHz. is not a system bottleneck, there are no real gains with faster RAM like in the old days with DDR and DDR2.
I personally recommend 1866 MHz. RAM for the FX processors as that is the default RAM frequency but 1600 MHz. will show no appreciable system performance reduction.
The quantity of RAM is dependent on your O/S and application software. With a 64 bit O/S you can use more than 4 GB. With a 32 bit O/S more than 4 GB. doesn't actually get used even if you install more. For most people 8GB. of RAM is just fine. Some folks that have software that can use more RAM opt for 16GB. The more RAM you use the more strain you place on the internal memory controller so more is not always better. 2x 4GB. 1600 MHz. or 1866 MHz. would be my recommendation. If you can actually use 16GB. then 2x 8GB. would be OK also.
Good brand name RAM comes with a lifetime warranty and you may need it the way I see newer RAM failing in a few weeks to a few months. Buy the RAM as ONE MATCHED KIT to prevent issues. Mixing of two or more kits even of the same part number has been known to cause issues for some PCs.
FWIW, you do NOT need top mounted RAM heatsinks with DDR3 RAM as it runs cool. The top mounted RAM heatsinks cause clearance issues with quality, large tower style CPU heatsink/fans that are required if you overclock your 8-core FX CPU or run it max load a lot.
Technical ignorance is NOT a destiny it is a choice. Do your homework so that you can make technically informed decisions and not be duped by advertising hype or mis-information stated as fact when its not.