My new phone plays MP3, AAC, MIDI and WAV files. All my music is WMA. If I convert, which form takes least memory? I'm guessing WMA would but that's no good!
AAC is the format to go for (probably - mp3 still more universal). if you have iTunes, you can easily convert all your wma files to aac, as long as they aren't protected.
if they are protected, you'll need to re-rip all your CDs and things.