Coming from a Florida school, I feel comfortable telling you that you should be fine wearing a dress you would wear to a graduation or a house of worship. Strapless or spagetti strap sundresses are appropriate, too, just don't look like a hoochie mama.
You don't want to go prom or semi-formal, but if you want to go for a manicure or fun hair style, go for it!
For dresses, Dillards or Burdines are great choices, and so is Ann Taylor. Most of the sororities will be wearing black, or a dark color, and they will be more formal than you. It's ok to wear any color you like-- black, white, pink, blue, etc., so long as you feel great in it! Again, comfy shoes! Kitten heels work perfectly. Leave the platforms in the closet this week.
When you have Orientation/Greek Forum, your Recruitment Counselor can better guide you. The counselors might put on a fashion show, and there might even be some tips in your recruitment brochure. Or you can call the office of Greek Affairs at your school, and a volunteer can also assist you about what to wear each day.
Here are some examples of things you could wear for prefs:
http://www.anntaylor.com/IWCatProduc...duct_Id=722248
http://www.dillards.com/webapp/wcs/s...06&rootCat=410
http://www.bananarepublic.com/browse..._category=2019
Good luck!