Both the GBA and GBA SP are backwards compatible with Gameboy Color and original Gameboy games. I recommend getting the GBA SP, the SP has a backlit screen which allows you to play it in the dark where as the GBA does not. It also folds to close which helps keep the screen from getting scratched. The only advantage the original GBA has over the GBA SP is a headphone jack.
The Gameboy Micro WILL NOT play Gameboy Color or original Gameboy games.