Co-op Food & Drink is a great space for large group private dining. It is located in the Hotel On Rivington and is completely separate from the rest of the restaurant. They will  let you pick your own menu as well. It has more of a cool vibe than an upscale vibe though.

Try The Modern...I think you can fit 60 people in their private room and the view of the sculpture garden is beautiful.  I've had several events there and it is always incredibly done.  Another great place is Aquavit.

I’ve made a list of some of the nicer restaurants in the city who are able to accommodate larger parties. I have put a star in front of the restaurants in midtown:
 
*'21' Club (American) (Private Room)

Benjamin Steak House (Private balcony area)
BLT Fish Shack (Private Room up to 125 for cocktails/ 65ppl for seated dinner)
BLT Prime (Steak) (Private Room)
BLT Steak (Private Room)
*Blue Fin (Seafood) (Private Room)

Bobby Van's Grill & Steakhouse - 50th Street

Buddakan(Asian)

The Capital Grille(Steak) (80-100ppl)
Daniel(French) (Private Room for 20-90ppl)

Landmarc(Private room for 125ppl)
*The Modern (American Contemporary) (64 max seating/ 80 standing)

*Ruth's Chris Steak House (Steak) (Private Room for 100ppl in their upstairs board room)

See more of my travel advice at http://www.TripKnowledgy.com

Robert restaurant on the top floor of the Museum of Art Design or the new private space at the recently opened Tavern On The Green.

 Nougatine At Jean-Georges