Motherboard 1333Mhz RAM 1600Mhz which should i buy to upgrade?

[Vucko94]

So i have 1, 4GB RAM stick manufacturer is Nanya Technology, speed is 1600Mhz, Cl11, 1.5V.
I would like to buy another 4GB stick but it's very hard to find the same manufacturer since it's very rare in my country, my motherboard is MSI 760GM-P23 (FX) and my question is can i buy another manufacturer memory with same stats as my current one because my ram runs at (full speed 1600Mhz) but my motherboard only supports 1333Mhz and has only 2 slots or is it better to buy 8GB one and sell this one?
Now my question is should i buy 1600Mhz ram or 1333?

 

[Tradesman1]

Regardless of what you find, get, there's no guarantee it will play with your current DRAM, best bet is to get a 2x4GB set