Maggiano's will bring giant salads and giant vats of pasta etc. They have GREAT cartering food and its perfect for a party of 25-30 people.
George Jewell has a new To Go catering www.georgejewellcatering.com they will do as much set up as you want and you can rent any supplies you might need from them too. Chicago Party Rentals is also a good place to get supplies.
Don't worry about juding the # of people when it comes to ordering pans of food--the place you order from can tell you how much to get for the number of people that will be eating. Be sure to tell them how many are adults and how many are kids! ALSO--I often plan to make a few of my own appetizers...this way people will eat something before the main meal...you will be sure not to run out of food---an extra few easy desserts along with the cake (brownies, chocolate chip cookies) has the same effect.