Hi, I am a complete noob when it comes to this stuff. I've looked at a few different options but I'm not sure what i want to go with yet. this is what i want.

either an atx athlon 2000+ system with 512mb ram, and a 250gb 3.5 hdd onboard video. ( I think this may be overkill for a carputer)

(this is the part where I need help)
or a mini itx or mini atx system. (what kind of quality are these systems?)
the only pc's i build are with asus mobo's so i have been concerned about these.
either way i still want the 250gb hdd though, i have a lot of music and movies

i want the carputer to start up when i start the car and shut down when the car turns off.
I just need a set up that wont freeze on me while watching movies. I may hook up a gps in the near future also.
what would be the safest most stable system, at a resonable price?
Thanks for the help in advance