All RAMs will run at the speed of the slowest. So regarding performance it is better to have the same speed on all dimms. The improvement is not that big though. More RAM will have a greater impact.
For DDR3 1333 and 1600 Mhz are typical speeds. The x2 comes from the Double Data Rate. Wikipedia says:
"Describing the bandwidth of a double-pumped bus can be confusing. Each clock edge is referred to as a beat, with two beats (one upbeat and one downbeat) per cycle. Technically, the hertz is a unit of cycles per second, but many people refer to the number of transfers per second. Careful usage generally talks about "500 MHz, double data rate" or "1000 MT/s", but many refer casually to a "1000 MHz bus," even though no signal cycles faster than 500 MHz."
from http://en.wikipedia.org/wiki/Double_data_rate
Well this has been marked as a suplicate while I was typing and apparently I can no longer answer but if you want a pretty complete answer to your question, this is what I had http://markdownshare.com/view/a6658da4-6995-448b-8e87-2818c978cec0
– flungo – 2015-05-28T10:07:48.370