I have an ACER aspire 5733.  It came with both memory slots filled, one DIMM0 with a Kingston DDR3 1024mb PC3-10700 (667 MHz) and the other, DIMM1 with a Kingston DDR3 4096 MBytes PC3-10700 (667 MHz). 
 
I'd like to replace the smaller one with another 4gb stick so that it performs a little better, and is more likely to cope with Windows 10, but I can't work out how to idenitfy the right one to buy.
 
Which specification is most important to match. Is the Mhz important ?  Some UK sites don't quote a mhz number on what seem to be the right sticks.  how do I work out what pin I need?
 
any help most welcome