jac is correct. A ND filter will not solve your silhouette problem. You'll get the same exposure but with either a slower shutter speed or a wider aperture.
But on the subject of ND filters, read this entry on
ND filters to help clear some of the confusion. Some ND filters are labeled with a filter factor (eg NDX8, NDX400) and some by their density (eg ND.6, ND.9). The table in that link will tell you how many stops a filter is when given by the filter factor without knowing some math or by density.