In HTML Editor, I have a "Visual Editor" Tab. Does this provide the same functions as the "Visual Site Designer" product?

No quite... It has similarities, but if you are wanting to do you site with visual editing, you'd be better off with VSD. Be warned, that if you do use the Visual tab within HTML Editor, you SHOULD NOT switch back and forth between it and Coding tab. This WILL cause you to loose your hair (from pulling it out over problems) and think that you've gone nuts, your code WILL change on you.

That being said, if you haven't already done so, download the trial version of VSD and kick the tires best way to figure out if it's for you.