If you complete Metroid Prime with less than 74% complete, you get the worst ending, 75-99% you get the normal ending, and if you complete 100% of the game, you get the best game ending possible!
To play the original Metroid on your GameCube, you need to have the GameBoy Advance and Metroid Fusion. If you beat Metroid Fusion and link to the GameCube, you'll be able to play the original NES Metroid.
If you scan greater than 50% of the creatures and the research, you can unlock an image gallery.
If you scan ALL of the creatures and do the research, you unlock another image gallery.
If you complete the game with 100% complete, you unlock a "hard" mode.
To use this trick, you need both the GameBoy Advance, and the Metroid Fusion game. Once you beat Metroid Prime, you can connect the GBA to play Prime with the Fusion costume!