I am 5' 8" and 140 lb. I can get up to 65 and even 70 if you give me a couple minutes lol. However, 65 on this bike isn't the most comforting speed in the world to be at imo. By the way, for your sprocket question, I refer you to this thread.
viewtopic.php?f=9&t=4505
There have been many other threads discussing the pros/cons.