How Do I Record Video Games With HD Video and Audio?

If you've been a gamer, you would have at least looked up a few reviews or spoofs of video games on youtube. A group of friends and I have always wanted to do reviews of the games we play or random spoofs or shenanigans we please. http://www.youtube.com/watch?v=-fRgRzBiGrY Two Best Friends Play: is a perfect example of what we would like to accomplish tech wise. How do we go about recording these games in such a good quality and be able to have our voices put onto the video along with it's lowered game volume. I realize it must be done with either cords to a camera or software that tracks the video from an Xbox/ps3 to the computer. However I would really appreciate a fully detailed explanation on how to get these HD videos the way they are. Thank you very much.