I have several X-Fi cards at home. It would be nice to know what they sound like. Unfortunately I use Linux.
One major difference between cards and onboard is cards get a lot more effort into the tools and drivers. That alone could make the difference in what people are perceiving without requiring any kind of hardware or circuit quality difference.
I just moved an SB X-FI card from my old gaming machine to my work and listen to music machine and it DID make a difference over onboard sound. I did it to take advantage of some tracks that use surround sound and noted a difference in even Youtube (which is strict stereo) music. The card was already paid for and there so I figured why not...
Had I not played the same track on Youtube only a short gap to install the card and basic drivers I probably would not have noticed the difference. Back in "the day" that sound card with "surround sound" headphones gave me tremendous advantage in some games as I could actually hear distinct sets of footfalls in relative spacial position in game. You can't do that with two low-quality channels coming out of the game.