a song can range from 1mb to 5mb, depending on length and MP3 conversion settings.
most of my songs are 3.5mb, i convert in 192k (CD Quality) i have i think between 30,000 and 50,000 or so, and thats approx 465gb. last time i fired up Winamp it said my libruary if played continuous all songs back t oback it would last 37.5 days
i'd be surprised if you get over 100gb.
500gb external is an ok size, but 1tb are really cheap now so i would go for that, as it will be more useful for other stuff