I have done some thing similar.
Except I used desktop PC components (put in the trunk), installed a 7" touch screen in the dash (tried to make it as stock looking as possible), add some front end software to windows (to make it not look like windows, instead more userfriendly for driving and touch screen).
I use it for:
-Surfing internet (Using wifi, did not have my iPhone last year.. when the car comes out again in the summer I will use iPhone/3G

)
-Music, It replaces the cars stereo. So I play all the music through here and it comes out cars speakers.
-Movies, DVDs
-Games
-Engine management/Tuning
-GPS
Pretty awesome overall, and it only cost me around 500 or 600 to build (I had the computer parts from before).
Overall A LOT cheaper than stand alone system made by pioneer or Alpine (or other car audio companies)... Also offers a lot more features than those 300 dollar GPS units (plus looks nicer when done right).
Check out
www.mp3car.com to learn more (go to their forums if you want to learn about building your own)

.
It was probably the best thing I have done to my car.