The computer components sound fine. You don't really need to worry about a mobo w/ wifi built in, as you can always pick up a usb wifi adapter.
I've considered this before, and there are two... well not even problems, but decisions to make.
1. Power. You either need to find a 12v input power supply or, you need to run an inverter and a normal 110v PSU. Inverters are pretty cheap these days, but still, you have to buy one and find a place to install it.
2. Sound output. Your car speakers won't just directly hook up to your PC. You will need an amp. Then you can just run a 3.5mm to RCA cable from the PC to the amp.
Also, It won't be cheap by the time you get your computer, amplifier, inverter?, screen(s), gps reciever etc.etc. (crappy screens can be had for cheap, but decent ones are still pretty expensive)
Check out the Garmin USB 18xusb reciever. I have one and it works great.
Garmin GPS 18x Serial and USB GPS Receivers at The GPS Store
Check out mp3car.com They a store and a forum about exactly this.
I ended up just getting a top of the line Kenwood GPS/DVD unit with backupcam XM etc... I can hook up an ipod or memory stick to it with movies and music and control everthing on screen. Cost was about 2k CDN by the time I got the unit and all the adapters and such. I admit, a self built carputer would be cooler if done right, but it will probably take some time and money to track down all the parts and find a control software that you like.