For starters, why do you need a 1.6GHz machine with 1GB ram **in your car**?
The Opus won't power all that. Incidentally the 1.6GHz (0.18u) takes 63W (~13A) of power max, and the 1.7GHz (0.13u) takes only 50W (~10A) instead. In either case, the max the opus can pump out is 12A which in both cases will be too little. (especially if you keep the rest of the system like you describe).
Don't forget, not only can these things get stolen, but then you have to worry about powering such a beast, which will be difficult with a DC-DC solution. You'll need to go KeyPower to get that much power out of a DC-DC, which adds $100 to your previous cost.
I'd seriously save your money and buy something a lot less powerful, unless you plan on ripping DVDs in your car in your spare time.
And really there is absolutely no reason to have 1GB of ram in your car, if anything it would be better to have less, not only because of taking less power but also if you ever want to use hibernate mode, it'll take 1/4 the time to read/write 256MB of ram to disk than it will take to write 1GB.
I would seriously reconsider putting that kind of power into your car, why not just get something cheaper and save your money? Unless you plan on hacking RSA in your car, even a 500 will run movies and mp3s no worse than a 1.6GHz machine can, but with a lot less cost and power requirements. DDR is completely unnecessary in your car.