ratio

Hi there, I am looking to upgrade my cassette for the hills but I'm not entirely sure what my options are. On my Trek 1.7 I am running a Shimano 105 52-39 crank with a Shimano 105 CS-5700 11-28 cassette. My rear derailleur is a Shimano Ultegra 10 speed RD-6600, short cage I believe. (Not...