Another important thing about sound cards, especially the higher models, will help take a load off of the CPU. The sound card would be dedicated to sound processing, and with the X-FI Fatality and up models with 64mb of X-ram, it would be able to process even more sound effects. I think the new upcoming game Crysis is going to take advantage of that, especially since they claim there are going to be 20-30 effects PER GUNSHOT... shrug. I however, don't really see the point of laying down 200+ dollars unless your a true audiophile and want to be able to hear a cricket chirping behind you while walking through grass in a game... shrug.
Also, if you don't have good speakers, no need for a sound card, your just wasting your money. Most mobo's come with decent integrated sound, so your not losing much by not having a sound card...
UNLESS your big into audio creation, and I would then definitly recommend the Fatality or Elite pro by Creative. Otherwise, its pure overkill