Which ram should i buy for Samsung NP350V4X

[Vexxum]

I am new to ram stuff. My laptop is Samsung N/B SERIES 3 350V4X-SOCTH(also NP350V4X).
My question is what should i look while buying ram. (like speed etc)
At present my laptop had one 4gb ram and other slot is empty.
When i used crucial scanner it suggested me a 1.35V ram but my pre installed ram is 1.5v.
So i need your help.
Detail of my pre installed ram is in attachment

[GSKILL TECH]

Is your existing RAM DDR3-1333 or DDR3-1600?

Here is the DDR3-1600 model:

http://www.newegg.com/Product/Produc...-481-_-Product

Here is the DDR3-1333 model:

http://www.newegg.com/Product/Produc...-266-_-Product

Thank you
GSKILL TECH