np
DI box on wikipedia
It's commonly used in studios where everything is as modular as possible. For home usage, I guess, the more compact, the better.
I mean when you can have one gear instead of two pieces it's usually less messy to set the recording space up and in case something goes wrong you can, arguably, easier pin down what's the problem. Anyway, to sum up:
1) technically you can record guitar with any X-Fi
2) only Elite Pro has special guitar input that eliminates need for DI box.
3) Fatal1ty=X-Fi Platinum + XRAM. Treat for a gamer, not for musicians.
4) basic Xtreme Music version may be less comfortable to work with because it lacks multiple front inputs of the Platinum Version.
Hope it helps!