Upgrading RAM in Samsung NP-QX311-A01

Hi,
in laptop as in subject, I would like to perform RAM upgrade. Laptop has 4GB RAM: 2GB buit-in motherboard and 2GB in slot. This stick in slot has label:
Samsung 2gb 1rx8 pc3-10600s-9-10-zzz

In specs, I can see that max RAM is 8GB, but is it total or 8GB max in slot? Also which RAM should I buy? What type and what timings? (remember that I cannot replace 2GB of built-in stick)