I just installed a 17t up front, and 42t in back. The bike is much smoother starting off in first gear, and cruises at L.A. highway speeds at about 6k rpm, which is virtually vibration free. Better performance on both ends of the transmission, for less than $100.
I wanted to go +1 to correct the speedometer. You can go to this website to estimate the impact on rpm at selected speeds with different gearing combinations. It is pretty cool and pretty easy to use.
I used Marthy's 16/38 as a model.
Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator