You'll find lots of threads on this subject and lots of opinions too.
But basically, there is a formal night, for which some rent tuxs and a semi formal night. Disney asks that no shorts or jeans be worn at dinner but on Tropical night, people do wear shorts and hawaiian shirts.
If you have a tux or would really enjoy dressing up, go for it. OTH, you can wear a nice suit or dress for both formal and semi formal and wear a sports jacket/tie for Tritons. (Tritons is dressier than the other two resturants.) Nice slacks, polo shirt, sundress, capris, etc. are all good for the other two restaurants. Also, if you decide to eat at Palo's, you'll want to dress up a little.
Take less clothes than you think you'll need. You need dress up clothes for about two hours everynight, unless you want to stay in your nice clothes for the evening. Lots of people bring a couple pairs of pants or skirts and vary the shirts worn with them.