Upgrading cassette - what do I need to be aware of?
As far as I'm aware, Shimano and SRAM cassettes are interchangable, at least in the 8 and 9 speed range. Your derailleur should be fine.  As far as the chain goes, depending on how worn it is, you might want to replace it with a new one. Mixing a worn chain with a new cassette is a sure way to wear out the cassette prematurely.  

Also, this is a good upgrade, as long as you don't do too many hills, as you'll be losing a few of the easier gears.  I recently did a similar switch, from 11-32 to 12-23 with an 8 speed groupset and noticed a huge difference. You aren't making as drastic of a change, so I'm not sure if you'll notice it as much as I did, but having a single step between adjacent gears makes finding the right gear quite a bit easier.