I haven't done that yet, but my fear is you will download the music to the hard drive, and it will play them in alphabetical order according to artist name, instead of the shuffle option that the I-pod has. 
Your suspicion is correct although the alpha order it plays in can be selected by artist or album or music type...not the greatest interface but this is not an either or issue, you can always add an iPod...You can test it out by playing straight from a USB drive.
If you do upload MP3s get your music organized on the PC 1st, changing it in the car is a PITA....DAMHIK.
