Many people come to SF to see the sites (Golden Gate Bridge, Fisherman's Wharf, etc.). I think that is a waste of time. They're just tourist traps. You will find exactly 0 locals at FW (maybe the bridge and tunel crowd but that's it) and the only locals going to the GGB are using it to get to the North Bay. My advice is to stay away. If you absolutely, positively must do the touristy thing then do the bridge or better yet, do Haight Street. FW is terrible.
Instead I suggest you visit Union street and Chestnut street. Filmore street is also lots of fun. The South of Market area is great for bars, clubs and some excellent hidden restaurants. My wife and I like Irving Street. We call it the poor man's Union street. You also have Golden Gate Park nearby.
SF is about the food. It's all about the food. I'm going to say that again just to make sure you understand. SF is all about the food. Unfortunately the best food is expensive as hell (Boulevard, Bacar, House, etc.). If you can afford it great but if you can't then you are in luck because there are a number of excellent inexpensive alternatives as well:
Gordo's - The best burritos on the planet, bar none. They have several locations around the bay. In SF they have one on 9th Avenue right off Irving Street (and half a block from the park). Cheap as hell.
Ebisu - very popular sushi restaurant. Burt Wolf actually featured it on his TV show. Not exactly cheap but worth it. Arrive early and you can avoid the line. It's located right up the block from Gordo's.
Nan King Road Bistro - Best chinese I've had in the city (that is not dim sum). Cheap as hell and guess what? It's up the street from Gordo's (next block up). What I love about this place is that it manages to avoid the greasy food experience you are likely to get from most of the hole-in-the-wall "authentic" chinese restaurants in SF.
PJ's Oysterbed - Best cajun food in the bay area. AWESOME! not cheap but not too expensive either. They have a website. Look them up. Also right on Irving Street.
Truly Mediterranian - on Haight street. the chicken shawerma is fantastic. Falafels are fantastic. Cheap as hell. They have a second location on 16th street right across from Ti Couz, yet another excellent restaurant (crepes).
Cha Cha Cha - Great cuban restaurant. Come for the food. Stay for the Sangria. Great big pichers of it. Near the end of Haight street. A block from GG Park.
I could go on and on but I'll leave it there. I suggest you stick to the city. Two days is a very short amount of time and you don't have time to waste on a long trip to Berkeley just to visit a game store. Chances are that anything that you are looking for Gamescape (which by the way is near Haight street) will have and then some. Berkeley is great and deserves a day all on its own but come back some other time. It will still be there.
Lastly, I was just at Gamescape a couple of weeks ago. Unless they just got rid of all of their used stuff recently you should be able to find quite a selection.