Flipflop hub means you can install a freewheel on the other side of the wheel and run that way, and you can always flip the wheel back around to fixed. LBS should have them and do the install. You should try it fixed first. But anyway more teeth in back makes it easier to pedal, as does fewer teeth in front on the chainring. And vice versa. This moto track comes 46x15 fixed but you might see how that gearing is for you and decide to bump it down a notch or two to 46x16 or 46x17. Chicago's pretty flat eh? It's all about the "gear inches": fewer gear inches equals easier/faster pedaling and vice versa.
http://www.sheldonbrown.com/gears/