Latest X-Fi cards handle 128 voices simulanously each with reverb, occlusions etceters; that's twice as many as Audigy 4. When playing e.g. Quake 4 it does make audible difference. There should be more games using this technology coming in future, i think there are only few titles that fully support X-RAM.
I do not have Audigy 4 to blind test it but if I select Open AL (that means all of my X-Fi is used) in Quake 4 sound is way better then plain DirectX, every single sound gets real and very precise, I mean you instantly know the direction Stroggs are coming from.
I agree Audigy 4 is probably better value compared to X-Fi.
Still X-Fi is best performance card plus it comes with bery good surround sound for headphones. I got it and have no regrets about it sofar