Hopper, if you can get your 8-speed cassette apart you can exchange the spacers and use your existing cassette & chain. Your lbs might have or can get a 9-speed spacer kit (Depending on the quality of your cassette & chain, this might be a lot less expensive), or better yet the lbs or a friend might have a worn out 9-speed cassette they would give you and you could remove the spacers from it (Then it would be no cost). As has been said, your existing derailleur will work fine. Good luck!