Crystal

I would check these guys out: www.mini-box.com They have some really nice looking PC power supplies for automotive applications. You will be able to get rid of the inverter. This will save you weight (and thus gas) and inverters have quite a bit of power loss. It would be interesting to see how much power loss there is from 12V -> inverter -> PC power supply. A lot of PC supplies only get 80%.

Also, you can probably get your 5V supply directly from the PC supply, unless the PIC's are on when the PC is not.