HELP with video cards

I just recently bought a new computer at fry's
http://www.frys.com/product/6944247#detailed

-I want to upgrade the video to a AMD Radeon HD 7770 1GB, but the descripition says 300 W. I was wondering if i could still change my graphics card to this, even with 300 W. If not, please tell me what graphics card i should get. I want to play games like League of legends or Skyrim at least mid-to high settings and at 1920x1080 resolution .

I still haven't opened my computer yet, so i dont know whether i could replace or add a video card. I dont really know alot about computers, so please help me.