There are very few memory bound tasks for modern CPUs. The key factors to look for are capacity, cost, latency, voltages and speed, usually in that order. It doesn't really matter if a CPU doesn't support a RAM speed. You just need motherboard support and go into BIOS to activate the XMP setting, so that it'll run at the rated speed rather than the SPD speed.
Without any further details about your specific workload (which is very important), my general recommendation, besides not to get a Z170 board unless you really need the extra IO, Smart Sound, or want to get a K CPU (overclocking), is to get 16 GB RAM straight away. It is very easy to use up 8 GB of RAM if you do more than one thing at a time, or if you're using anything that uses a lot of RAM. The price of DDR4 has dropped as well, so it is reasonably affordable to get 2 x 8 GB kits, in fact, cheaper than 2 x 4 GB kits of DDR3 last year: The G.Skill Ripjaws 4 series has a 2400 MT/s 15-15-15-35 kit for $74.99 at newegg.com. G.Skill is one of the largest manufacturers of RAM, and has a reputation for good quality. Their Sniper series generally has slightly more headroom, but it's just going to shave a few seconds off a 10 minute compression or video rendering, to name some memory bound tasks.
Basically, "overclocking friendly" just means that the factory clocks are slightly less aggressive, and that you can increase them a bit more while still being stable. The PC* ***** naming is just a different way of naming DDR* ****. The number directly after the PC and DDRs is the version number, and the second number is the bandwidth in MB/s for the PC scheme, and the clock rate in MT/s (twice the real clock rate) for the DDR scheme. Since DDR RAM has a 64 bit wide bus and there are 8 bits in a byte, the PC number is just 8 times the DDR number.
It doesn't really matter how you upgrade, just keep in mind that using memory from different kits may be unstable. The chance is low if they are the same model, but it's still there. Since you only have 4 RAM slots though, I'd just get a 2 x 8 GB kit and another when you decide it's not enough. Good luck.
Addendum: On memory timings
Memory timings tell the CPU how long to wait before expecting an action to be completed on the memory chip, and in SDRAM (clocked + refreshed RAM), is measured in RAM clock cycles (which is half the DDR number). Memory is arranged in rows, which are divided in to columns.
CAS Latency determines the amount of time the CPU to wait after issuing a read command before actually reading the data from the output pin. If this is shorter than the time it takes for the RAM to respond fully, what is read is a voltage that is transitioning between the previous voltage on the output pin and the desired one. This is bad and sometimes causes the computer to crash.
But CAS Latency is the full latency only if the right row is already open! If there is no row open, the CPU has to issue an Activation command first, and wait. This is the next number, tRCD, the RAS to CAS delay.
The third number is tRP. The Row Precharge time is the time you need to wait between a Precharge command and an Activate command; the time it takes to close a row, if you have the wrong one open.
Finally, the last number is basically CAS + tRCD, and then a bit more. It's the minimum amount of time a row can be active. There's a bit more again to let the memory finish passing on the voltage.
