asdfg wrote:
I have an MP3 player capable of playing AAC format as well as MP3. All of my
music in currently in MP3 format.

Should I convert my MP3 files to a slightly lower bitrate AAC? Is 96kbps AAC
any good? - because if I converted tracks to that I'd get a lot more on my
player.

If by convert you mean reencode from the original CDs or WAV files then yes, it would probably net you better sound or less space (maybe both depending on the rates you chose and the particular MP3 codec.)

If you mean convert starting with the existing MP3's I'd pass--the multiple compressions increase your chances of artifacts.