Go to "My Xbox" tab and the first screen is where it shows you what is in the DVD drive. Press "Y" and it will ask if you want to load it on the hard drive. The advantages are pretty great IMO. There is no loud ass DVD drive going, the screens load alot quicker and if you have issues with some games locking up like my buddy did with one game you this will allow you to play without freezing up.
I loaded Fable 2 and it took 6 gigs I believe so if you only have a 20 gig then plan on 1 game at a time if you have any amount of content loaded on the drive already. Also you still need to have the game in the tray to play it. I guess its an anti-piracy thing.