The 14/28 is the number of teeth on the smallest and largest cog of the cassette. From your description you want to make at least the second number smaller, possibly the first number.
As long as your replacement says that it's Shimano compatible (and 7 speed), you should be fine. Count the teeth on the cog that has the most teeth that you actually use...make sure the new one has at least this many teeth. I would go a few extra teeth but my legs get fatigued now and again and I just go into pleasure-commute mode. My off the cuff recommendation would be to look for a 13-26 (Amazon has these for less than $25 bucks). I'd say the 13-23 would be too much of a jump.
More information on the topic from the oracle Sheldon Brown: http://sheldonbrown.com/k7-7.shtml
Depending on how many teeth there are on the most largest cog, you may need to take a link or two out of your chain. Replace the cassette first and see (or better yet, take this opportunity to replace your chain as well).