I'm running a similar setup, only my all-in-one nettop takes 19V DC. I purchased a DCDC-USB power supply. It takes power from Battery and IGN lines and gives you a clean line of any voltage of your choosing (I forget the actual range, but 12V is one of the presets for it). It should handle the mobo and the monitor no problem. The output plug is a 4-pin molex plug, which can be easily cut off and split into 2 round DC power connectors (one for monitor, one for mobo)
Visteon Radio is designed to work without a clean DC power source - it can be hooked up directly to Batt / ACC lines
I'm not sure how to go about the amp - I'd have it powered from the battery over a 12V relay that gets triggered by the AUX power line from DCDC. AUX power line can have variable timing, so you can set it to turn on 20 seconds (or more) after the ignition - that should give your computer enough time to boot and prevent any thump.
I wouldn't hang the monitor, the amp and the computer on DCDC-USB - I'm pretty sure it's not gonna be powerful enough. If you get Opus 2x90W, one line can go to the amp, the other one can power the computer and the monitor. I don't think AMD Fusion board with an SSD and a couple of USB devices will eat more than 40W. Monitor is about 10W max, so you are looking at 50W on one of the lines. Amp will probably eat about 40W total, so Opus should do just fine.