This is one of those tasks that starts as a fun project and grows tedious when you realize you are only in the G's and the question of whether or not to keep Georgie Girl seems to matter less and less (I mean, I love the movie, but frankly, the whistling is on the wrong side of annoying–you know what I mean?).
So I actually recommend doing this chore for a couple hours, but over several days. The amount of things I don't know the who, what, where, when of how it ended up on my machine is amazing with all the songs I have been given and downloaded. Sometimes it's good, nothing beats discovering a great song, but it goes the other way too, and before you know it your either stuck listening to the bad kind of whiny indie rock or some horrid club music and you can't get the space on your crowded train to maneuver a change.
It is an awful experience and requires a good iPod cleaning, even if it means sifting through 3000 plus songs to do it.