I'd definitely start with sealing the house up as much as possible. There are some good guides online. I really like this one:
http://www.energystar.gov/ia/partner...e_May_2008.pdf
That is by far the best bang for your buck improvement you can make.
Next, I would look into getting insulation into those walls. The green and cheapest option is pretty much blown cellulose in the walls and attic. They cut holes in the wall and blow it in and pack it in a bit, and just blow it in in the attic.
After that, I'd probably consider a
mini-split heat pump like Xringer and a few of the other guys here have. Your house is relatively small and I bet it could easily cool down the whole house to a comfortable level without the larger cost of central air. Xringer also completely documented his install, so you could do most of it yourself if you're handy at all.
Passive solar is great, but tweaking your house after the fact isn't exactly easy. Window awnings or a large overhang to shade from direct sun would be a great addition. Interior storm windows (link in my signature for my project) will help as well in summer or winter to add r-value to a window.