I'd suggest doing a lot of web searching to get the basics of the question answered. Wiki has a nice chart that shows light transmittance and such, it can be found
here. There are several photography blogs out there that talk further about the filters and their uses. I have a Hoya NDX400, certainly it would be helpful to also have a graduated filter so some of the brighter parts of the scene don't get washed out (if you wanted to shoot a waterfall with some of the sky above showing).
I think the thought behind the smaller stop ND filters is the ability to stack them up to get different effects, and shoot at different times of the day. With the NDX400, I find myself exposing the sensor for 400sec or more, if it is getting close to dusk or dawn, you may only need a 2 or 3 stop filter and still get the same effect a 9 stop filter could give you at noon.
Hope that info helps, I'm also a noob.