If you're running a non-nvidia motherboard chipset and non-vista system, the X-fi cards are the next generation from Creative. However, the difference between the X-fi and the 2 ZS (a great card) will be nominal, unless the game is encoded for the X-Fi (like I think Battlefield 2 and other games).
For the X-Fi
Now, if you're running Vista, creative's drivers are still crap and will leave you with problems (on all creative sound cards). If you have like an Nvidia Nforce chipset on your motherboard, you'll run into crackling problems (not sure if it was fixed by a driver yet) - you can google this problem as it is extensive.
Otherwise, if the price difference between the X-FI and 2 ZS are similar, take your pick based on what I said above.
I've been rocking to my original Audigy Platnium for about 6 years now. Oldest part in my comp, but never had a problem with it (knock on wood)