I just got my Asus Xonar STX in today, and ho...ly...crap. This thing is crazy!!!!!!!!! I can't believe how much I was missing out of songs!!! I think it's one of my best purchases I've ever made. Theres just these little slight sounds that just pop out of the music now that I've never heard. The bass sounds a lot tighter now, and the vocals are sooooooo clear! It makes me want to listen to my music collection again.
One problem I had, was at first getting the headphone jack to work. What you have to do is go into the little control panel that comes when you install the drivers, click on the little triangle, and set one of the options to "Headphones" instead of "Speakers".
That leads me to the discovery of the Dolby Virtual headphone thingy . When I first choose the option, I thought "thats weird, the sound is now coming from my tv." So I take off my headphones, but I don't hear anything. So I put them back on, and realize that it's the surround technology. . It doesn't sound as good when you have the surrround on, but it's very cool.
So conclusion, if you're thinking about buying this, or you don't have a sound card and have always used onboard (like me), don't think about this, just buy it! Not kidding, it sounds so crazy awesome it's not even funny (well it was when I turned on the surround, but...). BUY THIS NOW.
Edit: I completely forgot about gaming! This thing is crazy awesome for gaming. I played an hour of Bad Company 2, and wow. You really feel like you're. The biggest difference from onboard with gaming, was the effect of being engulfed in sound. You could hear if there was a car coming up behing you, but not just be able to hear it, you could hear how far away it is from you. Or if a helicopter flys above you, you can hear how far away that helicopter is away from you, and you can also determine how high it is. I haven't tried gaming with the Dolby virtual thing though, it kind of late here. I'll try it in the morning though.
But don't know how BC2 can make a difference because of this