I m assuming that you are talking about main street. There are several restraunts on south main such as Louis and Clark, the Vine and the Little Hills Winery. Not every one is open on Sundays for dinner so you may want to check. You might want to look at www.stcharleschristmas.com to see all the festivities that are going on in old town for the Christmas season. One close hotel is the new Embassy Suites that has very nice suites at a reasonable rate. You are also just a few blocks from the Ameristar Casino which has restaurants, entertainment and a hotel too.
I don t know any really upscale restaurants, but if you go into downtown St. Charles, there are so many fun things happening along historic Main Street. There are a ton of cute shops, carolers, people dressed up as holiday characters. It s a lot of fun :) maybe not exactly the fancy romantic setting you re looking for, but it s really cute and I think you d enjoy it. There s a couple restaurants along Main Street as well, such as Lewis and Clark s and the Trailhead Brewing Co, which are both very, very good but not really as high as your price range. Make sure to call ahead and make reservations fast, because a lot of restaurants in downtown St. Charles are crazy on the weekends, especially the two I just mentioned...
The Ameristar Casino has very nice restraunts
This guide with user reviews should give you some ideas