With your budget:
1. Sansa Fuze+ 16GB w/ a microSD card for the expansion slot.
2. Creative Zen X-Fi Style (available in either white or black)
Have no experience with either of these current units, though I do have the earlier Fuze (no +) and it worked fairly well and the original X-Fi (worked decently). Both should be compatible w/ m4a files (my units are).
For a little more the 32GB Zune becomes available to you. I really like mine, though after a couple of years of consistent use the battery is starting to go on it.
I'm looking into getting an mp3 player and would like some input on which one. Some factors:
--I'm open to any brand (ipod, microsoft, etc.)
--I currently use iTunes for my music. I'm willing to switch to another program if necessary. Much of my music has been ripped from cds to my computer in m4a format. Only a few songs from the iTunes store (which I understand are only usable in iTunes)
--At the moment I have ~13gb music and 5gb audiobooks. Which rules out a 16gb player (I was really liking the ipod nano 5th gen. oh well)
--I'm not interested in apps. The only thing I might be remotely interested in would be internet/facebook capabilities. But it's not a requirement.
--I really just want a player to hold all of my music and books and have space for more. Would mostly be used for general listening, out on walks and maybe runs, and in the car with some type of fm transmitter.
--FM radio might be nice, but again not a requirement.
--Preferably under $200