If you look in my sig, you will see my build thread, which I need to get around to finalizing.
Anyhow, I have a 3 ghz core 2 duo running in my rig, runs everything that is playable that I want to play.
I switch between the xbox and the PC using a KVM switch that I attached to a green seimitsu button. Here is what it looks like:

You press the button and it switches the controls, sound and monitor to the xbox 360
If you have any questions, let me know