Erica,
Did you receive Elements when you purchased your XTi ? Are you pretty familiar with Elements ? Many of the same items will be used in CS2, CS3 ... however, the photoshop versions have much, much more than you can imagine. But, like everything else, there is a huge learning curve ... I know I am far from being even a mediocre photoshop user, but my knowledge has grown over the last 2-3 years I've been using various versions of Photoshop. So my suggestion would be based more on what you want to do, and how much time and effort do you want to spend learning the software application. There are classes at community colleges, and there is a website:
www.photoshopuser.com (NAPP) that has seminars, national conferences, tutorials and numerous books to support you in your learning. Even if you don't become a member of "NAPP", the next time a seminar comes around to Arlington (about once a year), put aside the $100.00 or so and plan on attending. You will be amazed not only in the number of people attending but also what the instructors will show you (and they only "skim" the surface).
Good luck with your decision.
Randy