I've been looking at all the methods of having all the cabinet hardware power up when the PC starts. They're great ideas, but I found an even easier way. There are several PC power supplies that have power pass-thru for hooking up monitors, etc. so they power up when the PC does. There are a couple I've seen under $30. One of these is going in my cab PC.
There is also an adapter I got at Fry's that turns the power passthru jack into a regular 3-prong power outlet ($5). Plug a power strip into this, and you're ready to rock! Now the entire cabinet (lights, speakers, monitor) powers on or off with the PC. Just check the rating of your devices to make sure the power supply can handle everything and you're all set (most can handle 3 amps or more, which should be plenty.)
If you're using a TV in your cab, you might not realize it, but most modern TVs will power on automatically if you pulled the plug while it was on. All 3 of my TVs, my VCR, and stereo do this as well. If your TV doesn't have this feature, I guess you'll still have to make a timed relay circuit for the TV power switch. But at least this solves 90% of the prob.