What media can I legally include into my videos?
Preface: I am not a lawyer. These comments aer based on my personal experience navigating copyright law. 

The general rule of thumb is that you can only use media that you have created. 

If you have not created the media, it's best practice to get or buy permission to use it, and get that permission in writing as a release form.

That being said, something like IGN daily fix could likely get away with demo-ing games under fair use as part of a review or demonstration. There's also the fair use exemption for something "newsworthy". There's a little bit of leeway when you shoot the video of the game yourself rather than capturing the output from the console. There you start to get into derivative work.

Personally, I would get releases for everything. In the case of IGN, I'm sure they can go to EA, and EA is more than happy to provide amterials for their use. You can do the same. Ask for a press kit that includes video clips. Those are licensed for that type of use.

If you don't have a license, here's what you risk. Let's say Google wants to buy your videos and feature them for something. Their lawyer is going to ask for chain of title documentation. This is signed, verifiable contractual agreements that say you have permission for everything. Sign a release form for you own voiceover jsut to please the lawyers. People will sue Google because they've got deep pockets. The lawyers need to mitigate that risk by making sure every i is dotted and every t is crossed. Are you willing to potentially lose a deal because you didn't get permission to use a clip? I've made the decision NOT to risk that...but that's me.

As many people as I know who are under-the-radar enough to get away with a lot, I know enough people who've been "made an example of" that I don't want to play with that lit match. 

I'd recommend a.) talking to an attorney and b.) asking permission for everything. You'd be surprised who says yes to the latter.