I am inclosing my garage and making it into a entertainment room. It has two outlets know one of them is for the garage door opener. The lights are on another circuit. I the cicuit will be running a big screen plasma TV and stereo as well as some table lights. I have bought 2/12 romex and was thinking I should upgrade the current circuit from 15amp to 20 amp

15 amp is fine for the TV and stereo.