Only an answer to a specific part, but as far as needing a car goes, in SF you don't, but you likely will if you plan to visit people/companies/events in Silicon Valley. It's possible to get to something like DevHouse (http://superhappydevhouse.org/) by public transit, but it's often a hassle. If it's only occasionally, though, you could sign up for a service like City CarShare or ZipCar.