A couple of suggestions -
Brio near Southpark Mall (Italian, steaks, seafood, accepts reservations), Barrington's (East of Phillips Place, would be at the higher part of your range, small restaurant that accepts reservations, great food and service, Zagat top rated), and Villa Antonio (Ballantyne Village - accepts reservations, Italian, steaks, seafood).
A dessert suggestion is Salara Dessert Lounge and Bistro in Ballantyne Village - cool place with high tables or leather chairs to hang out. Every kind of dessert you can imagine. If you were already in Ballantyne, it is easy to combine with the dinner restaurant for a special event.