Ahhh,,, reviving old threads to give my two cents.
I own an Audigy II sound card, and play a LOT of music on a set of custom bookshelf speakers and subwoofers. (nothing too fancy, but my tweeters can hit 30khz, and my subs respond nicely down to 33 hz (though they wobble about at 5 hz or so, 33 is really the limit for sound production)
anyway. I would never go back to MOBO sound. For one thing, its less stable. Having cpu hangups and glitches during a song STINKS.
For another thing, there is more capacitance in the card itself, everywhere. Moreso, the capacitors on a card can not only be applied anywhere they MIGHT be needed, they can also be generously sized and tuned to produce the best sound possible given the hardware.
There are of course many sound cards better than my little audigy, like ASUS Xonar, and SB X-FI, but that's not the point.
The point is, motherboard designers are forced to take sacrifices and make cuts on a sound card where design constraints interfere with what might be a miniscule benefit. Sound CARD designers do not have to worry about such things as much, and therefore will always be able to produce a better sounding product than the motherboard sound cards of years in the future.