Seems to be cost effective to go with burned DVD's. But a hassle if you're gonna use navi more often.
Everyones recommending the Avox jukebox but it's a toss up to me but if you'd see yourself using an mp3 player daily I'd go with the ipod because it can be used more often outside of the car also.
You'd lose your touch screen controls. The Avox is like a portable HD with more HD space. The cost depends on the size of HD you decide to get, but the avox alone is 100 at the cheapest?
The ipod you get touchscreen controls and better portability as an mp3/video player. The video quality may not be as good but that may depend on compression. Pick up a new ipod for like 225$ or used for less.
Burned dvd's... 10-15$ in a 50 spindle at your local fry's electronics? but live with swapping out navi and music/video discs.
BTW: a place to download videos and music, you should try learning how to use torrents.