Originally Posted by koine2002
It should probably work. Most medium cages, if I recall correctly, are up to 30, but I could be wrong. Be sure to get a new chain as well, because you'll need it 2 links longer than what it is (assuming it is already the right length), and it'll be less wear on the new cassette.
I second the 'get a new chain' recommendation - don't want a worn chain messing up that nice new cassette, especially a pricy one like a 970 or 990.