i think anything since the live! 24-bit series is only marginally better and not worth the price. you can pick up a live! 24-bit for ~$10 from creative on ebay. that's a good deal. the audigy 2 zs platinum has the front i/o panel, if i am not mistaken, which is important for sound recording. so if that's your thing your card does the job. x-fi is all hype, imo.
well, if you're just looking at creative then i've just run through their shortlist. so... yeah stick with what you've got because it's not worth upgrading, especially given that the next step up is x-fi and the price isn't justified for the marginal increase in sound quality.
integrated sound is good enough for most people, but i use a sound card because it offloads some of the processing to the card thereby improving cpu performance slightly.