With your budget:

1. Sansa Fuze+ 16GB w/ a microSD card for the expansion slot.

http://www.amazon.com/Sansa-Fuze-MP3-Player-Black/dp/B0041G62VU/ref=sr_1_12?s=electronics&ie=UTF8&qid=1311474979&sr=1-12

2. Creative Zen X-Fi Style (available in either white or black)

http://www.amazon.com/Creative-Style-Video-Player-White/dp/B003G4T4P4/ref=sr_1_2?s=electronics&ie=UTF8&qid=1311474979&sr=1-2

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.