I think it depends a lot on the plants you plan to have. Plus, the more plants, the more nutrients will be needed. According to my Encyclopedia of Aquarium Plants book, sand contains zero nutrients for plants, hence, you will have to add them all, with the exception of what is absorbed from the water (tap water). Flourite alone is high in nutrients, but fine gravel over the top of that is better, from what I have read, not from a nutrients stand point but from an upper substrate layer perspective. Have you considered putting sand under Flourite? Roots will take hold even better than with just Flourite.
Without going into a lot of detail at this point, to have plants in sand you must provide for a way to have the sand turned over on a regular basis or risk the health of the plants, as it is susceptible to compaction. A browse through the tank pics in some of the forums here, along with those under the Members menu item, should give you some ideas. I would read the Sticky's in the Plant forum as well for more information.
