U can try Desire at Scarlett Hotel, one of the more romantic restaurant in SG at one of the best boutique hotel. It's next to Maxwell Market (which you can bring her there also, the food is great!!)
For a more toned down setting with good food, i always reccommend Senso at Club Street. One of the best italian restaurant in SG. Not very exp also, less than 200 for 2 w/o wine.