What are the key differences between cheaper sound cards and of those in the steep end?
It mostly comes down to two things:
1) Signal to Noise ratio: anything above 95db doesn't sound like crap, 105 and above sounds great in a normal setting, for sealed headphones you might be able to tell the difference as high as 115db
2) Integrated Headphone amps
So, most of your super high-end sound cards are made for super high-end headphones. Other than that, it's up to your own personal preference. I have three computers (work, gaming, and a htpc). I have x-fi's on the two that are hooked up to surround systems, and a xonar dsx hooked up to the one with stereo.
The asus does sound a little better, out of the box, but I tend to think that's because it colors the sound more than the x-fi cards. But the x-fi's are much more tweakable, especially with surround systems.