suedonim, is the 3 of you you, dh and a child? What age? I have spent several weeks in San Francisco, usually a week there combined with a week somewhere else. I hope to get back there soon and find out what it's like with a new baby.
You should be able to book somewhere to stay at fairly short notice, I first booked into a Travelodge branch with a few days' notice but have returned there regularly with much more advance booking. I would assume though that August will be more expensive than September and February/March, so would have a look sooner rather than later.
Particularly at the moment, if you book direct with hotels/chains online you'll probably save money because of the exchange rate between pound and dollar (assuming pounds are what your income is in). You'll also gain flexibility, if you change your mind about what you're doing you can cancel direct bookings up to as much as 24 hours without financial penalty.
I use tripadvisor to get some sort of picture of where I'm booking, there are always people who hate places I've found acceptable but I've always preferred to spend money buying books than on luxury accommodation. tripadvisor also has city forums including a lively one on San Francisco with lots of info.
If you have two weeks, I'd recommend considering a day or two in Monterey, about 150 miles south of SF. It's a small coastal town, you can walk around and look at seals and other wildlife, go for meals out etc, there's an aquarium just up the road, or there's nearby Carmel which is touristy though I found it kitsch and preferred both central touristy Monterey and the more residential part of town with its secondhand bookshops, the town just made both dp and myself feel quite relaxed. Neither of us drive, and you could get there by a shared airport shuttle for about $20 per person in Sept 2005 (quite reasonable imo) and get around the immediate area by public transport.
Back im SF, there are touristy ferries and ordinary ferries that you can use to visit other towns across the bay.
I quite like just wandering round the different areas of SF - we've been to some of the tourist attractions but never made it to others and only once gone to some.