Thanks for all the help! (and JanMM, thanks for the math lesson, d'oh!)

I was considering just losing my left shifter, but if I did switch out the rear derailleur, are there ones that have a larger range/ratio? What kind of ratio should I look for to give me a wide range of sizes over 8 gears, to cover some hills and some power?

I'm still working on figuring out my current ratio with the Sheldon Brown calculator...

Thanks again!