I haven't been to Yank Sing in some time, though I remember it being very good but expensive. I've had a great and not too expensive dim sum experience at Gold Mountain (Broadway near Columbus).
There are already some good recommendations for very upscale dining on this thread. Here are a few more casual but still fantastic dining places:
- Coco500 near the ballpark is fantastic for California cuisine.
- Nopa (Divisadero and Hayes) is my favorite neighborhood restaurant, though it seems that the whole City is dining there every night.
- Delfina on 18th St. in the Mission has consistently fantastic ingredient focused Italian food.
- Zuni on Market street is good (search this forum for more info).
- For classic French food in a nice setting, I like Jeanty at Jacks in the Financial District.
You can also find some good leads by reviewing/searching the dining section of sfgate.com. I personally find Michael Bauer's reviews to be spot on when it comes to the better places in SF.