XBOX Live Gaming
Windows 8 will now integrate with the XBOX Live ecosystem allowing you to login with you XBOX Live ID and play games. The Gamer Points you accumulate on the PC will add to those you get on the XBOX with same ID. You will also be able to purchase games and interact with friends through the service, exactly the same as on the XBOX only now on the desktop.
The XBOX Live games you buy and download on the PC will load and play from the Metro interface. For most people they are put of the Metro start menu, but I think its nothing. Once you start using it, you won't even notice the difference. Your games will load and play like they always have.
Windows 8 will also come with the XBOX Companion App which allows you to control your XBOX console from the PC. Like the app already available for the Apple iPhone (My XBOX LIVE - http://bit.ly/NoCp3N) and on Windows Phone 7 (http://bit.ly/MoN3Mo), you are presented with a screen with buttons on the controller. This system uses your network and wifi to connect to the XBOX 360 and control it.
PC Gaming
Those on the PC who have invested their time and money with other PC services such as Steam and Origin will be able to play like any other system as if it was any other Windows 7 system. Ignoring the Metro interface, you will be able to install your games as normal. While future releases of these services might accommodate for the Metro interface, however I wouldn't hold my breath for that.
All-in-all, Windows 8 will allow you to continue to play the games you have now while also giving you the XBOX Live services to expand and connect with other XBOX Live players.
