Aliasalpha: You could get away without it but you tend to get better sound quality out of a discrete soundcard. There was a thread a while back with people discussing it, can't seem to find it, the general consensus was that soundcards take a small amount of work off the cpu and delivers better quality
oldschool: Too small to even be noteworthy. I've only seen one game that supports XRam, and it doesn't work properly. I used to be a Creative fanboy but their drivers really suck. One other point about descrete sound vs onboard, it only matters if you have a 4.1 speaker setup or larger imho.
*reads this right after mucking about with X-Fi Prelude driver packages and ALchemy in order to get Thief 1/2, Deadly Shadows, and the mic input all working properly*
Yeah, Creative needs to sort out their drivers big time. It's not as bad as it used to be, but still...
Nevertheless, I still won't do without the X-Fi. Not after getting my Audio-Technica ATH-AD700s. These cans with CMSS-3D Headphone give me some downright astounding surround imaging, enough that I don't really want 5.1 or 7.1 speakers any more-and I can only imagine how much better true audiophile-grade cans would make it.
Once you get used to it, you might even start getting accused of wallhacking...