There's Pro, Elite and Arcade. They all play exactly the same games etc but
Pro = "default" = 20Gb Harddrive, Component HD cable, comes with a Headset
Elite = "High end" = Pro + Black, 120Gb Drive, Component and HDMI cable and HDMI audio adapter
Arcade = "Mainly offline", no HDD, (has 256Mb Memory card), no HD cable, just standard RCA's (can upgrade to a drive later) no headset.
Components are interchangeable between then and you can buy the Harddrives separately, BUT, it's definitely more cost effective to buy the one you want upfront.