Slightly North of Broad or S.N.O.B. is supposed to be one of the best. I tried to go for brunch one day when I first moved here but it was closed. I guess the Penninsula Grill and the Charleston Grill are supposed to both be excellent as is the Oak Steak House. FIG, although I've only read reviews, sounds like it is very over rated. It evidently serves plain (frankly bland sounding) foods and charges an arm and a leg for it. High Cotton, I did go to and it was very nice. Magnolia's is owned by the same people and I have not been there yet. I made a point to go to Hymans during a visit before moving here for the seafood because it had gotten so many great reviews by national publications including the New York Times and I was not dissappointed. The she-crab soup was the best I've tasted yet.
Restaurant Reviews, Hymans Seafood, Hymans Restaurant, Charleston SC, (http://www.hymanseafood.com/reviews.html - broken link)
I have not made it out to The Woodlands yet but I intend to because it is not only one of the best restaurants in Charleston but one of the best IN THE WORLD.
Woodlands restaurant, 5 star, Five Diamond, Charleston sc fine dining (http://www.woodlandsinn.com/dining.cfm - broken link)
Also a former chef of the Woodlands now has his own restaurant on Daniel Island called Sienna, so that is probably on my list of places to check out.
The Ocean Room at the Sanctuary on Kiawah Island would be another "upscale" restaurant for sure. I had lunch at the Sanctuary and it was good but not out of this world,,, but for some it might be worth a trip... just so you can say you've been there. The atmosphere is outstanding however and the service was impeccable.
Blossom, Cypress (also both owned by the High Cotton Owners)
82 Queen St.
Carolinas
Cordavi
I'm sure I'm forgetting several.
There are a few that have gotten inconsistent reviews that I left out. There are many many more restaurants that are not so-called "up-scale" but are considered among the best restaurants and have excellent, even gourmet food and give you a GREAT VALUE for your money. (I didn't include any of these because you asked for "upscale".)That is one of the nice things about Charleston, you don't have to spend an arm and a leg to get unforgettable food like you have to do in many other cities.
The Post and Courier has a good restaurant section
Restaurant search
That should get you started.