The audigy 2 ZS is one of the first cards worth buying since the live!
The audigy 1 sucked, they lied on advertising about its capabilities in regards to 96hz sound and 24 bits. Audigy 2 they finally implemented the stuff they claimed they were going to. It also has good quality outputs and inputs.
If you have a decent pair of 2.1 speakers it might be noticible in the quality differance from a live! just depends on what you listen to. If you listen to low quality mp3s, I doubt you'll notice much differance. If you listen to cds or high quality mp3s you probably will. Even games will probably sound better with audigy 2 zs as opposed to the live! because it supports a later generation of EAX. So you'll get the improved enviromental sound stuff, but not the improved surround stuff.
As for comparing this to the new x-fi, it just depends on what you want. If you primarily game, you may want to grab an inexpensive version of the x-fi card (as of yet there's not many games that use the x-fi game stuff, one is battlefield 2 though). If you like the platinum features of the 2 zs such as the box thingy with additional inputs and outputs (like if you are into recording stuff or mixing, or you are a musician) then you'll probably be needing those features more than a bit of gaming improvement.
You should read some good reviews on the audigy 2 zs (doesn't have to be platinum) and see if you like what features it has. And read some on the x-fi as well if you are interested in that.
A quick google turns up this very detailed review on 2 zs:
http://www.digit-life.com/articles2/creative-audigy2-zs/
And on the same site a large 2 part on the x-fi:
http://www.digit-life.com/articles2/multimedia/creative-x-fi.html
http://www.digit-life.com/articles2/multimedia/creative-x-fi-part2.html