Indigo -
Well, it's not devoted to the subject of gold, but the Wells Fargo Museum has quite a bit of it and about it (that whole Gold Rush Connection, you know). It's in downtown SF, and yes, they also have at least one stagecoach set up.
Hmh, let's see, there was also an exhibit called "Gold Rush! California's Untold Stories" (commemorating the 150th anniversary of the discovery of gold at Sutter's Fort) at Oakland Museum, with the info still on the web...http://www.museumca.org/goldrush/
Have Fun!
Mary Kidwell
Museum Studies Grad Student, JFK University