The Avedon at SFMOMA is absolutely amazing, and there until 29. There's other great photography there at the moment.
49 Geary has quite a few small galleries with loads of great stuff. My particular favorite is Stephen Wirtz where I enjoy looking through drawers full of Michael Kenna prints.
There's also Fraenkel (sp?) in the same building.
Rayko on Third St. sometimes has great stuff.
If you have any questions feel free to PM me. Have a great trip!