Both Form-Z and Sketch up are fine programs... I use both of them on a pretty regular basis. Form-Z is great for super high quality 3d renderings, while sketchup is great for getting 3D concepts out of your head quickly and on to 'paper'. ArchiCAD is the best for drafting purpose, and you can export to Form-Z for finishing of renderings. Also Artlantis works very nicely with ArchiCAD 'finishing' interior renderings. Are you going to be using this for commercial stuff?? If not look into bit torrents...