Do not buy a rig from cyberpower, alien, etc. Unless you don't care about spending a premium for not having to put it together. Even then I would advise against it. They charge a sizable premium over putting the parts together yourself.
Start here: http://www.build-gaming-computers.com/
http://pcpartpicker.com/
If you can put together legos and follow the instructions in the above link you will save a good chunk of money. My first build took me about 2 hours to go from parts in boxes to windows installed and ready to rock.