I'll tell you what I did. First get one of these, or similar:
http://www.amazon.com/Smart-Strip-Protector-Autoswitching-Technology/dp/B0006PUDQKIts a smart strip/surge protector. One device is a "master device" that controls the other switched outlets. What happens is that the circuitry in the strip monitors the draw on the power for the master outlet. When you turn a device on, it draws more power from the outlet, the strip senses this and turns on the other outlets. So, you plug the computer in to the master, plug the monitor, speakers, marquee light, powered usb hub, etc. into the switched outlets. You turn on the pc, the strip sends power to the other outlets, bingo, everything is on.
Modern PC's use a momentary switch to turn on. If you open your case and follow the tow wires from your power button they will attach to some pins on your mother board. When your PC is off there is a constant 5v available there. when you press the power button on your PC, you close a circuit and short these pins, which sends a signal to the pc to turn on. Conversly, if its on and you hold it for 5 seconds, you close the circuit and send a signal to the pc to do a hard shutdown. To make things nicer, most modern motherboards have a power management feature that allow for a one-button shutdown of windows. If you go into your control panel, under power management features, you can assign what happens when the power button is pushed. If your motherboard supports power management, you can set your pc to either sleep, or shutdown windows when the button is pushed. So, instead of holding the button down for 5 seconds to shut off your pc or going to the start menu and hitting shutdown, all you do is tap the power button and the pc safely shuts down windows.
All you need to do this is a momentary switch and some wire. By the way, a momentar switch is any switch that makes momentary contact. The momentary switches you are probably most familiar with are the microswitches in your arcade buttons. Just wire the common and normally open sides of the switch, and presto, instant, cheap computer power button you can mount anywhere on your cab that will turn on and shutdown your entire rig with one button press.