go for 49x18. Go ahead and try these other combinations but in my oppinion if you are coming from a geared bicycle you will not appreciate these ratios. Some people like to spin and have leg speed which in my oppinion is quicker, and easier on your knees than mashing big gears. Trust me, you will always feel like you are chasing the gear. With enough practice you will be comfortably spinning down 9th Ave at 30 mph (140rpm) Lastly, if you have any desire to race track just throw a 15 on the flip and you are good to go.

Not inferring at all that the other posters do not know what they are talking about. Find out what works for you, but just don't be like one those kids walking a 2000 dollar fix up the Williamsburg Bridge.