Little story. My first X-Fi sound card.
I had been relying onboard sound on my nforce board because it was able to do DDL Live 5.1 encoding for games on the fly, meaning I could use surround sound. That was all fine, but then I upgraded to another motherboard. That meant I had 'HD' Audio and no longer used my receiver.
I randomly bought a X-fi fatal1ty fps, and didn't seem to notice any improvement in sound. I had that card for a while.. then sold it. I went back to the onboard audio and was horrified at the sound quality. I just had to buy a new card. The difference had only become apparent when I had got used to what 'quality' was meant to sound like, and I noticed all the details in music/movies.
I bought another X-fi Fatal1ty FPS soon afterwards and did pretty much back to back tests of the x-fi vs the onboard. The difference in quality was very more than noticeable.. the music was just ace in comparison.. the onboard just sounded so bland and flat. (I was not using the 'crystallizer').
Since then I simply cannot be without a sound card. I have had another two x-fi's.
Another note.. is that during game play.. postional audio is MUCH better... and critically... the system doesn't slow down during heavy audio situations like explosions, or lots of guns fire/grenades like the onboard did/does.
The sound card would be the first thing I would cut from a system spec... but would be the first purchase as soon as I could afford one.
Also.. the better the speakers.. the worse the onboard will sound in comparison to the decent sound card.