I lived in downtown Milwaukee for 8 years and with the exception of a couple hills, it is exceptionally flat. I ran a 48x16 (go ahead and start talking about "skid patches" kids, I dont ride around skidding everywhere...) for a 78ish gear inch and had no problems. Felt right to me. Your mileage may vary..

Please see this URL to assist in your selections:
http://www.sheldonbrown.com/gears/

[EDIT]
oh and if you do start talking skid patches, it could be known that i *am* an ambidextrous skidder