that dress looks just fine. you will see people who are ultra-formal and those who are really just dressy-casual. i think you should wear something that you are comfortable with and that you think you look great in. (I also think that dress is gorgeous!)
that dress is very elegant looking. The only way you will know is to try it on and see how you feel in it. It might be a good idea to get a wrap, shawl or passima to go with the strapless- it can be chilly in the dining rooms and shows.
Unfortunately i wasnt able to access the dress u were gonna wear. However theres this great store called TORRID (they specialize in sizes from 12/14-26/30 nothing below 12) they have great dresses and arent pricey i got my dress for my formal night and planning on going there to get my formal night dress for this year's cruise thee one i have my on is about $50-60 but u can probably use it again for another occassion wedding or semi-formal (versatile) i know i will.
That dress is very pretty and will be flattering -- do bring something for your shoulders though in case it is chilly. Some people dress up more than others and you need to wear what you feel good in. 8)