tabsie
Don't fall for his smart ass joke, Ninth.
There's an EB Games on 151 Powell Street (in a mall; 415.362.7601) that may sell some PC games, new and used; call them first and ask. That store is right in the heart of the tourist section (at the cable car) so you're gonna be around there anyway. I know you're not a heavy console gamer but they sell a lot of good pre-owned PS2 and Xbox titles REALLY cheap.
There is a CompUSA store downtown on 750 Market Street (415.743.3200), towards the Embarcadero. They sell the more prominent stuff, nothing you couldn't otherwise get in Paris, but you might just find some goodies in their bargain bin.
There's a BestBuy on 1717 Harrison St. (415.626.9682), but it's too far from the tourist section, and besides, they pretty much also sell prominent games like what CompUSA would have.
There's also a Gamestop on 1501 Sloat Blvd. (415.682.7318) but that's waaaay too far from where you'd be
but they don't carry PC games, but they MIGHT carry PC games, new and used. I know the one I frequented in Chicago does, so call them first and tell what you're looking for. It may be worth a little side trip.