Re: overdrive and distortion?
An overdrive pedal will attempt to recreate the sound of an overdriven amp (think Stevie Ray Vaughan). Distortion will add "fuzz" to a clean signal. If you are looking to add more to your already dirty tone, get an OD. If you are looking for lots of gain and/or are a metal fan, get a distortion pedal.
If you have a tone you are thinking of getting, let us know. I'm sure all of these guys can help out.