X-Fi would be a better future-proof deal
But the main reason to get X-Fi (despite marketing claims about "Crystallizer") is that it uses a new chip that makes audio resampling inaudible. And if you do not want it - you can turn it off - unlike on most other sound cards.
Can get X-Fi Xtreme Music or X-Fi Xtreme Gamer for less then $70.
Full Vista support might play role in future as well, Audigy is kind of old now.