Dont know much about ram and speeds

Solution
It mostly depends on your board. What motherboard are you planning on using it on?

Ok so capacity is pretty straightforward- it is how many gigabytes of temporary storage you will have- generally, the more the capacity, the faster your system is

Having one or two modules depends on your board and cpu and if it supports dual channel or single channel better. Also, using single module will leave room for upgrades.

The frequency is how fast each ram module operates and how fast it can transfer data. The 1866 Mhz is faster than the 1600.

In your case, depending on your board, I would go with 2x4GB@1866 or 1x8GB 1866


But please post specs before buying to check compatibility.
It mostly depends on your board. What motherboard are you planning on using it on?

Ok so capacity is pretty straightforward- it is how many gigabytes of temporary storage you will have- generally, the more the capacity, the faster your system is

Having one or two modules depends on your board and cpu and if it supports dual channel or single channel better. Also, using single module will leave room for upgrades.

The frequency is how fast each ram module operates and how fast it can transfer data. The 1866 Mhz is faster than the 1600.

In your case, depending on your board, I would go with 2x4GB@1866 or 1x8GB 1866


But please post specs before buying to check compatibility.
 
Solution
It's best to have a 2x4 stick at 1866mhz cl9, that's assuming you are building a gaming pc .Because by having 2 sticks, the board can utilise dual channel, so instead of drawing the processing power from just one slot, the 2 separate sticks allow it to draw the power from both, `866mhz is better imo, there are tests that show that any speeds above 1333mhz don't make a difference, but 1866mhz at a latency of 9 is considered by many as the sweet spot for gaming,