My setup doesn't use composite and if it did even with 800x600 it would look very grainy. I use 800x600 VGA and when I had a mac in my car I used a program called AMP, it's like frontrow but has a ton more options and toys that come in handy for a carputer.

A power inverter will work but theres a DC-DC power supply for the mini for carputers. Just splice into the car stereo power or even go directly into the fusebox.
To live is to let die