i personally am a big fan of wma. this converter here can convert mp3 files to half the bitrate with the same effectiveness as the original mp3 file. i noticed no difference and was able to save over half the disk space.
http://www.mympxplayer.org/mp3-to-wmazip-df238.html
if you have files worth 2gb already on your player and you convert them to wma, you should be able to put over double of the files you have currently have on your player
on an average maybe around 650-700 songs on a 2gb player