Play around with this:
http://www.gear-calculator.com/#
Gives you a nice graphic showing how the range, spacing and cross overs compare with the different cassettes.
There's also Sheldon's calculator:
http://www.sheldonbrown.com/gears/
But I like the first link better because of the graphic display.
__________________
You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.