Currently Being ModeratedFeb 16, 2013 8:45 PM (in response to tartanmartin)
As long as you ordered the correct specs RAM, a total of 20 GB RAM is fine. Make sure you install the new RAM in the two unpopulated slots so they match horizontally.
Where did you order it? OWC (macsales.com) or Crucial?
Here are the complete specs for your machine - and it is a 2011 model (even though you bought it in 2012):