Hubert -
Will you have a car? How adventuresome are you about using public transportation to travel to outlying areas?
In San Francisco, a must-see is the DeYoung Museum. The photography is limited but classic, but its inspiring to see some of the paintings.
Another place we discovered when we were there two weeks ago is the Cantor Center at Stanford University in Palo Alto. There is some photography, but more importantly, they have a fabulous collection of Rodin sculptures. Rodin's work is very influential in my portraiture and figure photography. It's accessible via the light rail system.
If you have a car and wish to drive to Carmel (about 2 hours south), the Weston Museum and Gallery West both have excellent displays.