After seven cruises with Princess, I can assure you that there's no danger whatsoever of being "overdressed" on the formal nights. Since each seven-day cruise ordinarily has two formal nights, you might want to consider a couple different combinations of shawls or other accessories so that your outfit will look different if you have only one formal gown. Also, be sure to bring a cocktail dress for the semiformal evening in addition to the gown.
Your husband may wear a DARK -- and I empasize the word "dark" -- business suit on the formal nights if he does not own a tux. Additionally, he will need either a suit or a sport coat, dress slacks, and a tie for the semiformal night.
It's always a good idea to pack an extra semiformal outfit (basically, an extra dress shirt and an extra necktie for your husband and perhaps a different set of accessories for the cocktail dress) because there's sometimes an opportunity to attend a special event that requires semiformal attire.