Mirroring Option with AppleTV and iMac

I am interested in the mirroring option with the AppleTV. From what I have read, only the newer macs have the capability to stream to a TV using AppleTV. My iMac is a mid 2011 but is only running on Lion 10.7.3. If I were to upgrade to the latest Mountain Lion (10.8.5 I think) will my computer be capable for the mirroring option?

I do not have an AppleTV yet so I cannot test it out myself. I want to make sure I can utilize the mirroring option with my computer before I purchase.