A bit of advice, if you get a set of slot-in filters (ie. the rectangular ones, not the screw in ones) you can SOMETIMES, depending on the size of the filter, use a graduated ND filter as a normal ND filter. If the filter is large enough, you can slide the GND filter down so that all of the 'dark bit' covers the lens, so it is basically a normal ND filter.
So you have two types of the filters in one.
Be warned though, it does depend on the size of the filter. I use LEE 150mm filters, which are quite large. If you use Cokin, some of their filters are much smaller which may not cover the whole lens. AND if you use a soft edge grad filter it may not work as well too.
Just a thought......