Go to Sheldon Brown and plug in you present ratios into their gear calculator. I'm building a new road bike and am going to use a 50/34 crank married to a 13-25 cassette. That gives me 101 GI's at the top and 35.9 at the bottom. That'll do for me. I do group rides at a fairly sporty pace with my club and at 58, that setup will work for me.

Figure out what you need and go from there. Nashbar has a nice selection of cassettes. Al