Take a look at Sheldon Brown's gear calculator, to get an idea of gear comparisons http://www.sheldonbrown.com/gears/

I agree with Dan, that a 42/16, which with 700x23 tires gives 69" of progression is optimal for a variety of terrain/conditions. I've used 49/19, which gives 67.8", for years, and it's worked well for me, flats or hills.