There are a few things that I know from researching it you need a video capture card. After that, you render it in your PC and layer commentary over it through something like movie maker or more professionally something I have that I used when I was doing videos for WoW was Sony Vegas.
I am not sure how it works out right now, since I have yet to obtain a video cap card but I am thinking about doing that. This can get very expensive, you need a higher end vid cap card to capture hd gameplay, and if you really want to get into it, you can buy a mic and the whole shebang. I personally play on one of my monitor computers for the moment ( I have my tryhard pants on) so I would kind of suggest that set up so you can capture the video then move to rendering it and so on.