I guess it depends on what you are trying to do in your car. My 233 with 64Megs ram plays MP3s and runs the GPS simultaneously.
However it would really bog down with some of those fancy visuals y'all are putting on the monitor. (I can't hardly run the one that comes with winamp 2.67)
Speaking of that the one I like best so far is the default one on Win Mediaplayer 7. (I am sure there is one like that for winamp)