Prepare as much as you can ahead of time and enjoy your guests, they have come to see YOU!
Casual, business casual, semiformal, black tie and white tie are excellent terms to use. Making things up, however, is not the best idea.
Absolutely. A polite guest always brings a hostess gift. This doesn't have to be anything expensive or elaborate: freshly-cut flowers from your garden, a bottle of wine, a book she'd enjoy, or some chocolates all work well.
If the invitation says 8 - I'm an advocate of getting there at 8 pm sharp. Don't be early, unless the host has specifically told you you'd be welcome-usually, that's when they're running around trying to finish everything, and they won't appreciate having to entertain you. And don't be late, especially for anything like a dinner party where food is likely to be ready at a particular time. If it's a "drop in sometime between 8 and 10 pm," then naturally, you don't need to be there at 8 pm on the dot, but in general, there's no such thing as fashionably late.