I bought an Audigy2ZS not too long ago, I really like it, sounds even better than the SBLive I had for years. I don't really notice a performance hit in games, but guess that depends on your rig and how observant you are, heh.
During one computer build, it came with onboard sound (some kind of realtek) and tried it for a while. While ok for general usage, I thought it sounded muffled, like there was 10 feet of foam rubber around the speakers, and went back to the SBLive. I definitely notice a difference in games. Footsteps, echoes, effects, all sound better/richer and at lower volumes, treble is less screechy, monsters sound scarier, all that.
As mentioned, if you have the speakers (or even a good pair of headphones) I'd get a seperate card. I'll agree that many cards are overkill for the average person tho - doesn't cost much for a basic one these days. Just my 2cents.