I would recommend getting a new cassette. They're designed to shift better with the specific choice of cog sizes and properly shaped teeth. Note that changing from 23 to 25 is less than a 10% change - if you want to really feel a big difference in the gearing you may want to make a more substantial change.
As Grumpy indicated, you should check your chain first to be sure it isn't worn. Putting a new cassette on with a worn chain will generally work fine, but it'll start wearing out the cassette prematurely. And you might well need a longer chain anyway - that just depends on how much extra length your current chain may have. You want to be sure that the chain will be able to go on the large/large combination without binding - even if you never plan to use that gear combination it's a good idea to check that it'll work since you might sometime inadvertently try to shift into it and cause damage if the chain is too short.