Dude the only thing you need is a Opus. www.opussolutions.com Nothing more nothing less. The PSU that Opus sells its absolutely the best. No worrys about anything. It will turn your pc on once it senses 12v on the remote line.
To hook it up its damn easy. It has three wires that connect to your car, and it also includes instructions on how to do it.
Yellow wire - 12v constant, get this voltage directly from the battery
Red wire - 12v remote, get this from your ignition switch, specifically from the accesory line.
Balck wire - is the ground, connect it to the chassis of your car.
edit: Forgot to mention, the Opus also has a jumper that connects to the jumpers on your motherboard for the power switch. This switch is where the lil magic occurs. It will turn ON and OFF your pc.
Dont even think about it twice, best PSU period.