Basically what you need is a video crosspoint switcher. Something with like 4 inputs and 4 outputs. That way input 1 can be routed to output 1, input 2 to output 3 and output 4. Or input 1 routed to all outputs.
If you're handy with circuitry, Analog Devices has a 16x16 crosspoint IC. http://www.analog.com/library/analog.../31-2/200.html. You could use some of the pins for video inputs/outputs and the rest for stereo audio inputs/outputs.
But if you're looking for a ready to go solution, then something like http://www.audon.co.uk/609.html might work for you.
Finally, A.I.M.E.E. is adding support for multiple monitors. Then you could just add a couple of video-to-USB adapters to feed the external DVD and XBox signals into the computer and let A.I.M.E.E. handle the monitor output choices. http://www.aimee.cc/modules/news/