Just done the same myself. If you have a large larder cupboard or fridge-freezer then the level of the top of your wall cupboards will be set by this. Otherwise it's down to you. Your kitchen supplier's will have details of the top height of any tall cupboard, plus the height of the wall units from which you can work it out. DOn't forget though if your floor slopes you need to allow for this.
Wouldn't worry about the floor units, any fixings for these will not be weight bearing so the plasterboard should support.
Tango