YES, PLEASE, PLEASE, PLEASE DO YOURSELF A FAVOR AND HEAD TO AMOEBA
while i'm not familiar with SF at all (drive to LA, and then ask again); there is an Amoeba in Hollywood. and, though it is apparently like twice the size of the two SF ones, it is HUGE which means that the others can't be that small...
they have everything used, some things very cheap, and i usually just wait till i'm at amoeba and then bring back like 348932874243 cds. the place is a music-nerd's wet dream.