There may not be that much noticable difference between earlier versions of EAX and EAX5. I don't game so I can't really speak to that. ...and I'm by no means purporting that Creative's SoundBlaster line is the only decent audio card you can get. The X-Fi is a good card, it sounds good and its powerful, and, being a SoundBlaster makes it widely supported by developers.
One can definitely spend less money...and again, if listening to music is what you primarily do the X-Fi might even be overkill, unless you're listening to a lot of highly compressed MP3s (where that 24-bit Crystalizer might have a pleasant effect at
less distortion than if manually applied similar equalization by say...iTunes). I noticed the difference in my SB Live! and the X-Fi. ...and to be honest, the SB Live! is not a bad card at all. Again, I've read that cards by the likes of M-Audio are just as good (if not better) for less money when it comes to music. If I were a gamer, though, I'd prolly stick with the SoundBlasters as I've experience with them and none of that experience has been bad.
The Audigy 2ZS and the X-Fi's (besides the $370 Elite Pro) share the same DACs but I think the X-Fi may have a better S/N ratio. ExtremeTech:
http://www.extremetech.com/article2/0,1697,1850365,00.asp
I don't think anyone's really saying the X-Fi is the best that can be had for all things. It's a good card and is indeed better than the $30 ACME special in terms of S/N ratio, distortion, DSP, and EAX support. ...but that's just evolution, really, eh? You'd likely get better sound by spending the money on more accurate speakers than on pairing an X-Fi with bargain basement speakers IF music listening is what you plan to do.