Considering that most people can't hear the difference between sound sources when the frequency response is 40hz to 15khz (or better) and distortion is under 1% --a spec. easily met by all modern sound cards-- it's unlikely that any sound system is going to actually sound much better than any other.
I tested this many times, back in my stereo sales days, by setting a blindfolded customer in front of two different high quality stereos, setting the volume and tone controls identically and swithcing them back and forth into the same speakers. The test subjects got it wrong more than 50% of the time, most wisely ended up deciding their amplifier and receiver purchases on features and price instead of trying to judge sound.
The real differences between sound cards are in drivers, features and midi players.
Good soundcard drivers are totally transparent to the sound. They simply pass whatever sound is being produced along to the sound chip. (with special effects turned off)
There is little doubt that the MS GS synthesizer is the worst thing to happen to midi music in a long long time. The distortion guitar sounds like a freaking duck call and the rest of it ain't much better! Sound Blaster products, beginning with the pci16 have always had the best midi players I've heard. (short of sending midi playback through an external keyboard, that is)
So long as the drivers are stable and transparent to the sound, Stereo CD, Wave and MP3 playback are going to sound about the same no matter what soundcard you are using.
The really big differences are in speakers. Most of the rinky dink little plastic speakers they sell for computers sound best when they hit the floor. Hook up to a home theatre or stereo setup and be ready to be amazed. Even a medium priced boom box (with an aux input) is going to sound better than those silly little speakers.
On a final note... I notice a number of motherboards coming out with TUBES (holy regression, Batman!). These are being used to <i>deliberately</i> alter the pure sound reproduction from your system. Aside from introducing even more heat into your case, like all tube amps, the frequency response is non-linear and the distortion will be in the order of 3 to 5%... Not a wise choice.
Hmmm... this turned into a bit of a rant... but I hope it's helpful in some way.
<b>(</b>It ain't better if it don't work.<b>)</b>