Another option would be to change your chainrings (after consulting your front derailleur's capacity and making a gearing chart to check it for being practical). But, a cassette change and perhaps a longer cage r.d. to take up the extra chain slack sounds better. You might make up a gearing chart anyway to check for redundancy and shifting patterns with your new cassette. Go to http://www.sheldonbrown.com/gears/ for an automated calculator.

*** Changing chains along with your new cassette is a good idea...you may well need some extra links anyway to accomodate "bigger" cogs.