i went in the middle of february and it was AMAZING weather but i think it was a bit of an anomaly -- it was around 15-16 during the day and got down to around 4-5 at night. generally i think you're looking at 10-15 degree highs though
check the environment canada website; they should have seasonal temps.
depends on where in san fran you're referring to ... anywhere close to the bay will get *very* cold and windy. a winter jacket is sometimes needed.
but if you're talking about the core or in the greater bay area it is generally nice during the day (light clothes, possibly t-shirts most days) ... at night it can dip down to close to freezing but normally not under.
i was just there in december and it was nice for the most part, a fall jacket was all i needed for the week at night.