(Wed 10:24 23/Aug/06)Assuming the subsoil is something better than just bare rock, turf: none, seeding: 2-3 inches.
buildersmate
(Wed 12:32 23/Aug/06)A cubic metre of soil weighs about 1.6 tonnes (dry). Having worked out the area and the thickness required, you can then calculate the number of cubic metres, and convert to weight (which is how it will probably be sold to you). A tonne seems a lot but it doesn't go very far when spread out.