Best microphone for voiceovers?

I'm looking for recommendations for microphones to use with Final Cut Pro's voiceover function.

I'm creating how-to videos using Final Cut Pro 5.1 on a G5 Mac, and I find myself doing a lot of voiceover narration for them. The microphone I'm using is a little lapel mic with a mini-jack that's plugged into a Griffin iMic, which in turn plugs into one of the Mac's USB ports.

So with this setup I can either use a USB mic plugged directly into the Mac, or a mic with a mini-jack plugged into the iMic.

The audio I'm getting from this tiny lapel mic is usually adequate, but often I can hear a little hum or whine in the background, and the words could be a little more crisp. When speaking I hold this mic near my mouth, but not directly in front, to avoid getting the sharp puffy p and b sounds and esses that you seem to get with any mic that's held too close to your mouth.

I suspect I coould get better results with a better microphone, so I thought I'd ask anyone else who might be doing frequent voiceover narration in FCP: what microphones have you found to be the best for this? Can a good microphone be had for $50 to $100 (I see some in that price range on Amazon.com), or does it take a really expensive professional mic to get good results?

Any microphone advice would be appreciated!

Tom