Your story sounds a lot like mine: same type of school, same majors, same time frame to decide to grad school. I did well in my undergrad math classes and went on for a master's in math. I applied to a smattering of schools, three in the top 10, another three in the top 20, and some nearby state schools -- 11 apps in total. I tried to split my apps between good programs and places where I thought I could compete for funding. I feel like I played it conservatively, though.
Apply to schools with faculty who do the kind of work that you'd like to do. The whole process seems like a crap shoot anyway, so why not roll the dice.