Barbetta on restaurant row has a lovely garden. It was great for the wedding reception I attended, which had about the number of people you're inviting.
Are there any budgetary restrictions? If not, you might want to consider Astra, located on the 14th floor of the D&D Bldg. During the day it's a cafe; however, in the evening, it's an event space. Beautiful views + an outdoor terrace. Charlie Palmer does the food.
