I have a 2GB sony MP3 player and it currently has 226 songs with a short video and some pictures on it with a bit more space to fill up. A 4GB memory card should easily hold 500 songs. Although I am not exactly sure how many it would hold.
actually it depends on the size of your music file which in turn depends on bit rate. Simply put, if your average mp3 file size is 5MB or so a 2GB will hold 2000/5=400 songs. 4GB will hold 4000/5=800 songs etc. Just divide GB which is 1000 MB by average file size in MB to get to the amount of songs. in other words GBX1000/Average file sige=number of songs. Hope it helps.