Best Restaurant Near City Center for a Long Romantic Dinner?
I'm thinking Lacroix in the Rittenhouse Hotel. They offer a 4 course menu for $75pp, but you will not be rushed out of there and service is excellent. With the view of the park across the street, it can be quite romantic.
Sample menus available here...
If you are not including the price of liquors and wines in your above, the Waterworks directly across from the Art Museum has fine food and very romantic views of the Schuylkill River and Boathouse Row.
They won't rush you and perhaps as an extra added plus you might want to peruse the famous Art Museum as well.
When my sweetie and I were first dating (now 8 years ago), one of our favorite romantic places was Friday Saturday Sunday (http://www.frisatsun.com/). Very cosy, maybe a bit "old school" these days, but we never had a bad meal or experience there.
Otherwise I definitely second the Lacroix rec.