I love cycling I've done it since I was five years old I've raced at the elite level, I've been a bike courier, I've even dabled in bike journalism. Now I'm going to tell you something that lots of people may not agree with here. You do not need a single speed bike. You can just set the limit screws on your deraileur so that you cant shift. Or even better yet have a workout where you purposely choose the wrong gears. If you really want a single speed to make you a better cyclist I would recomend a fixed gear like rob mentioned. The main reason is that it will force you to pedal way faster than you could on a free wheel bike when you go down hill. this is sometimes called overspeed training and it will make your stroke wickedly smooth and eficient. Riding up a hill in the highest gear you can can be done on a multigeared bike just as well as a single speed so why bother?