Do some reading.
I plan to get a mac mini and put it in dash. I would like to remove the factory head unit but what will I connect the speakers to?
get rid of those speakers and upgrade
get a four channel amp (50x4 max) should be fine
Probably the easiest way is to get an AUX input adapter and keep the HU. There are alot of advantages of having a HU as well as the PC in the car. If you cannot find the answer here (which you probably will if you search long enough) you may want to check http://www.davidnavone.com. They were very helpful when I was looking for an input adapter.
This thread will probably also be of interest to you since it sounds like you want to do the same thing:
"Protecting Privacy and Free Speech Online"
I'm keeping my headunit with my carputer
just for ease of use with the radio and cd player.
And I turn on the carputer when I want navigation or mp3/video player
also since my carputer is a laptop on a docking station in the trunk,
I canm take it out, and not loose tunes in the car