ayuluv.. again, i feel you should spend more time on grasping the basics first before you venture out to other specialised areas.
sure a ND filter can give you silky smooth water and dramatic clouds, but if you are unable to understand the basic exposure theory, it will be even harder for you to achieve anything from the ND filter.
most pple get underexposed pics when they first started out with ND filters and it can get very frustrating as to why you cant get a properly exposed pic even when you add 2, 5, or even 10 more minutes of exposure time to the already very long, long exposure.
anyway, if you are really out to grab a ND filter, just get the pro-standard 77mm diameter. use a step up/down ring if need be.