Heres the breakdown for the core and premium:
Xbox.com | Xbox 360: Customize Your Entertainment Experience
And heres the specs for the Elite:
Xbox.com | Xbox 360 - Introducing Xbox 360 Elite
Personally I have the 20gb, but the Elite wasn't around when I got mine. 120GB would be overkill for me since I dont download any games on my hard drive from online or store any music or videos. I wouldn't recommend the 512mb though. If I remember correctly its just a memory card so if you're playing Madden or any other game that requires a lot of memory you will fill it up in no time. Good luck with it.