I'm thinking about dropping my bike from 21 gears to 7, just because I find myself with a whole bunch of gears I never use. Can anyone suggest a way to figure out what the best gear ratio would be for me & my riding style? I know this sounds dumb, as "the sprocket you ride the most" is the obvious answer, but I'd like to find out if an alternate sprocket would serve me better. I have a 00-ish Specialized Hard Rock that I have put some street tires and a rigid fork on. I'm in Washington DC, which is pretty much one steady hill going down north to south (I live at the top, work at the bottom), and I use it mostly for urban riding on real crappy streets.

BONUS QUESTION: I'm thinking of hitting a thrift shop for a beater to strip down to a fixed gear. Anything special I should be looking for when checking out the frames? Anything I should steer clear from?