Zzzz , for above . sorry I am not expert but have a lil bit knowledge in IT .

Ram upgrade is the easiest among all , and not too sensitive .
Any brand any speed put inside ur laptop also will work .
If u dun mind just buy a 1600mhz pc3-12800 one . it will auto downclock to whatever ur highest supported speed .

And ...
Pc3-12800 = 1600mhz = 799mhz in cpu-z
Pc3-10700 = 1333mhz = 667mhz in cpu-z
Pc3-8500. = 1066mhz = 533mhz in cpu-z

Its kinda related to dual channel problem , if u times 2 u will likely get 1600mhz or so