Looking at the specs of the SB Z (I didn't find an Audigy z so assume you mean this cardhttp://www.newegg.com/Product/Product.aspx?Item=N82E16829102048
and the onboard, I think you will be hard pressed to tell any difference at all in gaming.
If you are useing top of the line audiophile headphones, and have really good hearing
, then the Z "Might" enable you to hear more of music.
Also, much will depend upon the speakers you are using with your PC or whether you are running the sound out of the PC and through a dedicated audio amplifier or receiver. PC speakers have NOT improved a lot in the last 8 years.
Down side of the Z is if you go all-out and do a 7.1 surround system, it doesn't mention that it is designed for that.
You already have the card. Try your system without and with the card. Let your ears help make the decision. Specifications don't really mean a thing until you hear the sound.