EAX

Well, EAX 4 is a 64 voice solution and EAX 5 (X-Fi family) is double that...128 voices. This is said to provide increased EAX performance. EAX 5 cards have a better ability to process 3D positional audio than EAX 4 cards, theoretically. This means the more complex the 3D audio in a given title the EAX 5 card can process the audio with higher quality faster than the EAX 4 card...this is one reason why X-Fi's are considered to be the best for gaming...not just because they're the newest. ...and all this processing is done in hardware on the card giving us the beloved Tank APU, the X-Fi's heart.

For everything you ever wanted to know about EAX and somethings you may not... ( http://en.wikipedia.org/wiki/Envir [...] extensions ).

A common misconception is that EAX IS the 3D positioning, and therefore EAX 5 has better 3D positioning than EAX 4 because EAX 5 is newer. Not so to my understanding. As I understand it the reality is that EAX 5 handles 3D positioning with better results and faster than EAX 4 meaning more 3D affects can be turned on because the hardware to render those affects is available.

Hope this helps.