I like 72 gear inches if there are some hills. I'd go a little higher if I lived in Florida.

Here's a handy chart you can use to figure current Gear Inches and see which changes will yield which results.

BikeCalc.com - Bicycle Gear Inches Chart

There are other handy charts there. Like this speed @ 90rpm chart:

http://www.bikecalc.com/gear_speed