i've thought about doing somethign like this many times, however i just dont have a real good place to mount the screen in my car...and honestly i dont think i would really get much of a chance to use it. i just dont find myself sitting in my car needing to play donkey kong all that often.
now, i have thought about adding a LCD screen to our minivan for the kids to watch movies, so there is a good chance that i will install a modded xbox when that happens.