In case you're shopping online here are a few things to keep in mind that might save you some headache:
If you go to a bigger cassette (27,28,29T last cogs) you may need a new chain (or need to add links to your current chain) to allow for cross chaining ("big/big"). Of course you could leave the chain as is and be very careful about your gear selection...
Depending on what rear derailleur you have, clearance between the upper derailleur pulley and the biggest cog can be an issue. Usually this isn't a problem until you are looking at 27/28 teeth for your biggest cog. If you go to the 28, and the pulley is running too close or touching the biggest cog with the p-tension screw bottomed out; you can flip the screw around (thread it in so that the head of the screw rests on the hanger) and buy yourself a few extra mm's of clearance.
A 32T cog won't fit most standard double road RD's. But there are options to make just about anything you want work.