Yes, you can run 2 x 2Gb.....
If this is your motherboard, it shows it supports DDR3 1333Mhz speed. You want to Match the other DIMM exactly if you can for best results.
http://www.ebay.com/itm/like/261371751398?lpid=82
Download and install "CPU-Z" The memory tab will show you what you have (or, just get the manufacturer and part number off the DIMM in the system).
- DRAM Frequency: 666Mhz (is most likely... could be 533Mhz too just double this number)
- CAS# : this will be the "CL" number when you goto pick out your new DIMM values of 8, 9, 10 are typical
- other values are Not important...