The upgrade is totally justified, imo. It's a better purchase than what I got. I got the model with the 64mb X-ram, which is supported by a grand total of 2 games.
I noticed that everything sounded better when I switched from the Audigy 2 to X-fi. Even poor quality youtube clips sounded clearer. I would recommend Bioshock if you want to hear a huge difference from the Audigy 2. The only drawback is the Creative patented pop, snap & crackle but I get them far less often with the X-fi.
Remember to download Alchemy for vista. It's necessary for conversion of EAX to Open-Al since Vista doesn't support EAX directly.