Three suggestions:
Cafe Baba Reeba has a very nice 15 person private room in the back and will give you a huge prix fixe meal for about 35 bucks a person, including hot and cold tapas as well as paella. That leaves plently of dough leftover for sangria and other libations. The room is set off enough to give you a bit of privacy, but also let's enough restaurant noise to enjoy the festive atmosphere.
For something more "adult", Brasserie Jo and Mon Ami Gabi also have very nice private dining rooms, and Brasserie Jo's wine room is especially nice because of the large windows that let you see into and feel part of the restaurant. Both places will provide a prix fixe menu, including wine, to fit your budget if you ask them to.
In case anyone's wondering - no, I don't work for Lettuce Entertain You. It's probably true, though, that I think LEYE places are better and more experienced than most at accomodating large-ish groups, and the three places above have darn good food too.