NoCO2. Welcome to the site. I just went through the same thing as Metro stated. I have varying commutes lengths and was looking for some way to close the gap between biking and the car. I went with the a conversion kit that has a plug and play more or less. I not very mechcanicaly inclined so it worked out well for me. If you look on the EM blog under Laz cycling log it will give you more detail.
I'm 160 pounds and with the motor only it tops out at 22 mph. I use it as an assist and not just as an electric bike so I've easily gone 25 miles and the pack still has a lot of life in it. With moderate effort I can cruise around 20 mph and taking it easy 17 mph or so.
Here's another thread on making a e-bike