So I have dumb question, when the dlc is available will it appear automatically in xbl games section or do I need keep restarting my system? I have never waited up for dlc or been so eager to download
It'll most likely appear FIRST on the xbox.com website. It's happened before. So you can just pay for it on the site if it's live there first, turn your system on and it'll automatically start downloading for you.
But to answer your question, you shouldn't have to keep restarting your system (probably not good on your hard drive either), just load an app, watch some Netflix or Crackle (free to gold members) on the 360 and back out every now and then to see if the marketplace updated.