you probably don't have any replies to this b/c the things you asked about are very general in nature, and would have to be answered based upon application, personal choice, etc.
Best thing to do is, before buying anything, look at your set-up and pick out the weak points, and either upgrade them, or be prepared to do so when you test out of the car.
Your hard drive, CPU, M/b, and memory seem pretty adequate. The vid card may be overkill but that is just one opinion. Trying to play games on a carPC is not safe, useful, or wise IMHO. The video card in 3d will draw a lot of power, and get hot. You'll have to overcome both of those things to do this, and I know the lack of power will be a biggie. Besides that a car is not the optimal place to play games. Where will you be putting your mouse and keyboard so you can play? Simple games like MS Hearts ya, but a 3d shooter, or any other 3d game, is not going to be very enjoyable.
Just my .02.
Narrow down your questions and place them in the appropriate groups will get you better answers.