Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Sounds like the setup of your monitor, your hardware, and the limitations of the XBox 360 all work out really well in your case. The monitor has one DVI-D and one VGA, right? The MBP can use the DVI-D; the XBox cannot, but it can use the VGA. So you can have them both hooked up at once. 🙂
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.