The iMic makes the sound you record into your Mac much clearer because it sends the audio over USB. The standard 1/8 inch jacks on all computers are notoriously noisy, as they pick up interference from inside your machine.
As far as microphones go, Radio Shack sells some decent ones, and you can check out Zzounds.com as well. For vocal work you'll most likey want a unidirectional mic. This means it only picks up what it is pointed at.
If you're willing to spend extra dough on more high quality gear, keep reading. You will also find an article on setting up the optimal recording enviroment at the bottom of this page.