The problem with a couple inches of topsoil layed over a clay bed is the flooding of the root zone... I don't recommend any root zone space of less than 4"... W/out first removing a couple inches of clay(more expense) it is likely that in most locations, your topsoil will be floating down the sidewalks on it way to the storm sewers at first real rainstorm... If the OP's post is in reality,,, sand... Does it DRAIN???