What's the best way to record game footage from an xbox, edit on a computer?

I've been wanting to make up some video game commentary, because i'm bored for the summer. The idea I have to a way to stream the game footage to the computer and use software to add voiceover, effect and whatnot. im using the iMac desktop, its about 5 years old, and the xbox i have is a second hand arcade version. Thanks