Also, here is a great bike gear calculator that will let you compare two setups.
http://ritzelrechner.de/?GR=DERS&KB=44&RZ=11,13,15,17,19,21,24,28,32,37,42&UF=2312&TF=75&SL=3.5&UN=MPH&DV=ratio&GR2=DERS&KB2=55&RZ2=11,13,15,17,19,21,24,28,32,37,42&UF2=2312
Also, here are the calculations for...