I really get the impression that the step from Audigy 2 ZS to X-Fi is not all that a dramatic one!
Thats the impression I initially had, and it was with some reluctance to move from a 2 ZS to the X-Fi XM, but I did and difference was quite noticeable. I was hearing things never heard before on the 2 ZS, especially in music, delicate percussion, cymbals, etc. Even gaming. Quite happy with my move to the X-fi.