Audio: Get all the basic X-Fi improvements. That's it.
You don't get EAX 5.0 or anything else.
Gamer: Plus EAX 5.0, which is all about surround sound.
Music: Plus tons of additional software to improve the sound, like Crystallizer, which is supposed to make mp3s sound as if they are CD's. Also, has a serial port (?!) and the ability to play 192kHz in certain situations (Digital to Analog I believe)
Fatal1ty Pro: Music + 64MB XRAM, which is supposed to remove the overhead for the CPU in processing multiple voices at once (really helpful for Battlefield Games).
Fatal1ty: Fatal1ty Pro + a front facing bay for lots of inputs.
Elite Pro: The bay is replaced with a box, so that you can have easy access to even more inputs outside the computer, and a remote's included. ASIO recording's included, so you can work with say Pro Tools with little problem. IMO, it's best to get a card specifically for recording like M Audio's Delta series, since they run cheaper and have better quality, if you don't mind not having the box.