The Singh-Ray VariND is not a graduated ND filter. Grad NDs are dark on one half, clear on the other. Since the sky will be 2-3 stops brighter than the ground/ocean, the idea is you place the dividing line at the horizon to darken the sky (sunrise, sunset) while letting the land/ocean be more exposed. That means screw-in grad NDs are not ideal, since they force you to place the horizon in the middle of the image (rather than off center, perhaps along one of the 'rule of thirds' lines, which is generally preferable).


So, you want a rectangular grad ND so you can place the horizon where you want. You then need to choose how strong (2 vs. 3 stops are the usual choices, look at some images you have shot and see if adjusting by 2 stops would be enough to properly expose both sky and ground). You also need to choose hard or soft - hard has a crisp line between the dark and light portions, soft is a more gradual transition from dark to clear. You'd use hard for a clean horizon, like sky over ocean, and soft for an uneven horizon, like hills or trees.


Holders are optional, the filters can be handheld - perhaps try it that way first, and if you don't like the results, get a holder. Make sure your filters and holder are wide enough not to vignette on your widest angle lens.


An alternative to using a grad ND filter is to try HDR to blend exposures - software like Photomatix is cheaper than a good grad ND filter (free to try, in fact).


Hope that helps...


--John