You really need to look around these forums.
I suggest starting here: New to CarPCs? Don't know where to start?
That's a newbie's guide to vehicle computing that answers many common questions.
There are also a number of informational threads, including many writeups about GPS software, receivers, integration, etc. in the FAQ Emporium: http://www.mp3car.com/vbulletin/the-faq-emporium/