It's actually more complex than that. Top speed isn't just limited by your cadence because wind resistance increases with the square of relative wind speed, so doubling your gear inches doesn't double your real-world top speed since wind resistance is now 4 times as great. Also, as you would expect, climbing becomes more difficult with a higher gear. You may have a mostly flat city, but if your gear is too high, you'll find out right quick exactly how many hills there are.

I comfortably cruise around 20mph on my 77" gear and can break 30 on the flat under my own power. But I don't expect to go any higher since I spend enough time starting and stopping that acceleration is important to me and maintaining higher speeds in a higher gear would be much harder due to the wind.

Sheldon has a nice online gear calculator. http://www.sheldonbrown.com/gears/ 67" - 80" is generally the FG for the street range. Most people are pretty comfortable starting out around 67 - 72.

edit: oops, @ 47t, my gear inches are much closer to 77. 78 was w/ my old 48t ring.